“Raining cats and dogs” I had not looked up before, but I don’t hear it much currently so I would likely associate it with someone of a specific older age range, or someone younger who’s been around (or who is emulating) older people. Even that small inference brings some depth to understanding what someone is saying, and the character of the person saying it.

As for “butterflies in my stomach”, that’s even more important as it’s a tactile expression, describing the feeling of nerves as though there were literal butterflies fluttering lightly in one’s stomach. The fact that at some point an english speaker decided to express the internal feeling as though it was something tangible that other people around them were familiar with, that’s (imo) some pretty fascinating cultural context and something that I see repeated throughout english. I also suspect that it happens in many (but importantly not all) other languages/cultures, which is also pretty fascinating.