Might help if you think about it in these terms.
You're at a store, and you're looking for socks. So you stop a clerk and ask him:
靴下を探してるんですけど。
(くつしたをさがしてるんですけど。)
"I'm looking for socks, but..." Which begs the question, but *what?*
"I'm looking for socks, but..." (do you think you could tell me where they are?)
The latter is the subtext. It's implied, right? Not actually spoken, but the clerk knows what you're getting at. This kind of thing happens all the time in English, right?
Well, it happens in Japanese too. The reason "けど" placed at the end of a sentence is said to soften the sentence is that it implies a follow-up or subtext that is not directly expressed. This type of indirect expression is not unique to Japanese, but it is one notable characteristic.
Another example, taken from your post:
"そうだといいんだけど。"
"I hope you are right about that, but..." (I'm not entirely convinced it will come to pass.)
To express such a doubt so directly would be borderline-rude. On the other hand, implying it with no other word than "but" is highly ambiguous. Any number of things could go between those parentheses.
"I hope you are right about that, but..." (I have no way of knowing, so I defer to your opinion.)
...would be an equally valid interpretation. See how the ambiguity makes it more indirect and polite?
edit: why in the world does previewing the post remove all spacing? pain in the butt...