uh - there are only benefits if you need a server OS. Yes, there's more flexibility in terms of setting up shares, but you can do that in the client OS with an add-on utility such as Sharepoints.
OS X server is the way to go if you need to host a local domain (whether OpenDirectory or whatever), wish to host multiple web sites, your own mail server etc.
For simple file serving - unless a good number of workstations are going to be connecting to it simultaneously, then it's overkill.
Basically, if you dont know whether you need it, you don't.