これも覚えといてね。

I believe the long form of the above sentence is これも覚えておいてください。

My question is why おく is used?
I understand おく is used when it refers to doing something in advance, leaving something in a certain state.
I feel its use is superfluous even though it could still apply the same in this sentence.
Perhaps I need to be aware of the practice of how Japanese people feel when saying this way. Perhaps someone could shed some light.

What could be the difference in meaning if it is written as これも覚えてください。

～ておく also has a more general nuance of doing something for future purposes, i.e. performing an action that will have some benefit down the line.

これも覚えてください is fine grammatically, but it kind of sounds like you're speaking in a vacuum. "Please remember this, too (not for any particular reason, but because I'm telling you to right now)." whereas 覚えておいてください has an added nuance of "Please remember this, too (because it'll serve you well later on)."

In English, this isn't a nuance that is necessarily expressed using a specific grammatical structure, but that doesn't mean it's superfluous in Japanese.

Indeed, when learning Japanese (a language completely different from English in almost every possible way), often you'll come upon structures, particles, phrasing, etc. that can't simply be translated 1-to-1 into English. At times like this, it's not so much a matter of understanding "how Japanese people feel" (which makes it sound like these are arcane concepts that only exist in the Japanese mind, something that isn't really the case), so much as grasping the underlying nuance (even if it doesn't have a clear and straightforward English translation) and internalizing how it's expressed in the Japanese language.

I got a bit philosophical there for a second (sorry about that), but I feel like beginning learners (through no fault of their own) tend to overcomplicate Japanese. If you can gradually learn to "de-mystify" the language (and asking questions like this is a part of the process), it'll go a long way to helping you in your studies. Good luck!

I've come to realise that Japanese language has a lot of nuances that don't usually exist with the English language.

I believe I have to also learn to understand the Japanese way of thinking too.

I knew that there must be a need in the language for it to be expressed in that way. I asked though I might not get extra information it would reinforce in my mind whatever it may be.

Thanks for your elaboration.

I didn't mean to be rude at all. Just trying to say we think differently and we have to learn your way of thinking because language is the expression of the thinking.

On second thoughts, ～ておく must be used a lot in the language since many actions would have some benefit down the line.

Incidentally, ～ておく has another meaning "to leave something in a state".
e.g.
暗くなると怖いので、夜寝るときいつも電気をつけておく。
I'm scared of darkness, so I always leave the light on when sleeping at night.

天気が良かったので、夕方まで洗濯物を干しておいた。
Since the weather was fine, I left the washing hanging out until evening.

The light is not on previously for a purpose, or the washing was not hung out previously for a purpose here. In this usage, it often can be rephrased with ～たままにしておく.

healer said:

I didn't mean to be rude at all. Just trying to say we think differently and we have to learn your way of thinking because language is the expression of the thinking.

On second thoughts, ～ておく must be used a lot in the language since many actions would have some benefit down the line.

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I just wanted to say for the record that I don't think you're being rude at all, and I'm sorry if my response made you feel that way. On the contrary, your questions are very much on point and thought-provoking and (just speaking for myself) I enjoy answering them.

healer said:

Just trying to say we think differently and we have to learn your way of thinking because language is the expression of the thinking.

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It's interesting that you say this, but let me just say for the record that unlike Toritoribeさん I am not a native speaker of Japanese. I've been speaking and working in and living with the language for enough time (twenty-some odd years) that I feel confident in answering your questions,, but if anything, please take that as an example that a native English speaker can internalize and make Japanese a part of themselves if they are motivated and keep at it for long enough.

～ておく is a construction that you'll findd

Thanks for the input, Toritoribe san.

～ておく in this case refers to some action done and the result of the action is left untouched for a period of time. This even contributes to the frequency of using ～ておく.

Though the topic of this thread seems to have been answered, sometimes I just can't help asking questions on the explanation and examples given.

Toritoribe said:

暗くなると

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The translation of the above is darkness but the Japanese version literally means "getting dark". The Japanese sentence seems to say being scared of getting dark, or the process of getting dark. However the intention seems to say darkness. Would it be closer to say 暗いと怖いので、夜寝るときいつも電気をつけておく? My grammar of using i-adjective with と could be wrong. I just go by my memory. The point is why we would have to use なる in this case while we are talking about the result of the process. Again this could be something like ～ておく, just the way it is, the way this bit of language came about.

jt_ said:

I am not a native speaker of Japanese.

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I can tell you living in a foreign country for such a long time I'm sure you would have been in good contact with them and of course their culture. Surely your thinking and habit would have been affected to some extent. I believe that language and thinking interact with each other, affect each other.

Having studied the Japanese language for a while, I can understand why the Japanese people speak English the way they speak and conduct themselves the way they do.

I take learning Japanese language as a hobby or leisure so I don't need to rush. I don't know when I could get to the level of interacting with Japanese people effectively. Just keep my fingers crossed you people won't get sick of me.

healer said:

Though the topic of this thread seems to have been answered, sometimes I just can't help asking questions on the explanation and examples given.

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There's no problem to ask any questions you have. It's always useful to solve questions for learning (or doing anything).

healer said:

The translation of the above is darkness but the Japanese version literally means "getting dark". The Japanese sentence seems to say being scared of getting dark, or the process of getting dark. However the intention seems to say darkness. Would it be closer to say 暗いと怖いので、夜寝るときいつも電気をつけておく? My grammar of using i-adjective with と could be wrong. I just go by my memory. The point is why we would have to use なる in this case while we are talking about the result of the process. Again this could be something like ～ておく, just the way it is, the way this bit of language came about.

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Yes, your understanding is correct. It indeed can be rephrased with 暗いと怖い or 暗いのが怖い. When I turn the light off, the room gets dark, so I used 暗くなる there.

As far as I remember, jt_-san never gives wrong explanations so far. His point of view from a non-native Japanese speaker is often an advantage to answer questions asked by non-native learners of Japanese.