To give feeling and emotion to sentences. If we did not put exclamation marks at the end of sentences, then the sentence may come off as dull or monotone.
For example:
I’m so excited for my birthday!
Look at the difference with no exclamation mark:
I’m so excited for my birthday.
Doesn’t that come off as not excited? Like you don’t really mean what your saying?
Lastly, it can also give a sentence or something a sense of urgency.
Like,
“ Help! My leg is stuck!”
Difference:
“ Help. My leg is stuck.”