A server does what it says on the tin in effect... it provides a service
A web server provides a web service, A mail server provides mail and so on... The question of do you need a server depends upon how you intend to work in the environment... I run a server at my home, do I actually require one, of course not.... So why ?

I use my server to share documents and media between all the systems which come into contact with it, it also allows the desktop systems to continue performing their best as the drives are not stuffed to the brim with files.
I also use this server as a web cache which vastly improves performance of the internet and a mail server scan and distribute emails to the correct machines....

The only person who can answer is a server wanted is yourself