Question

why would you use synonms in your writing

Answer 1

to make your writing a lot more interesting
and to make you seem smarter to your teacher lol

can I have a medal?

Answer 2

So you don't have excessive repetition within the same piece of writing.
I.e. The dog barked at the other dog for being a dog.
That's a boring sentence, but you can make it a little better...
The pit bull barked at the pelletzhu for being a dog.
Still horrible, but it sounds like you put some effort into it.