Seems to me basically what punctuation does, contact-us so why not kaomoji too?. When put together, they are characters that represent emotion, kind of like the exclamation mark. That contact-us does bring me to one other thing, which I think is pretty interesting though, and that is: Kaomoji As Punctuation. There are plenty of other punctuation marks in Japanese, but these are the main ones (or the ones that I thought were important to learn).
Bonus: Part Alternation Mark. It shows volume or emotion or contact-us both. The Japanese exclamation mark is used just like the Western one. contact-us Japanese Exclamation Mark.