When to use server actions?

Really confused on why and when to use server actions.

Can you guys give me an example when to use it? Specifically when sending a post and get requests on the server.

Also, How do I tell that there is new data in db and to rerender a component to show latest data? Should I just redirect to the same page to force it to render? (I dont know if this works).

Ps. im really new, I just cant get my head wrapped around it. Sorry in advance. Thank you.