X から Y んです. Why is んです necessary here?

Hi all.

I understand the function of んです as a marker of an explanation when it stands alone. What I don't understand is why んです would ever be needed in a sentence with から or んで. For example, what's the difference in meaning between: 食べすぎたから、おなかがいたいです。 vs. 食べすぎたから、おなかがいたいんです。Do they not both just mean 'My stomach hurts because I ate too much?'.

TY in advance.

The former is just a neutral description, whereas the latter is used in a context where it's an explanation as same as other ～んです, for instance, as an answer to the question 具合が悪そうですが、大丈夫ですか "You look sick. Are you OK?" or どうしてお腹が痛いんですか "Why do you have stomach pain?".