What is the difference between web api get and post

Question

normally we know for get request data pass as query string and for post data pass as form collection.

but in web api i have seen people use get when they fetch data from server side to client side and
when people send data to insert or update then they use post but i do not know the reason why.

so if anyone know can then plzz explain why get is used to fetch data and post is used to save data.

i guess we can code in such a way post can fetch data and get can be used to post data but we never do this why?

please share the reason in details. thanks