Fixed Server Roles are only necessary if the user in question needs to access Administrative permissions on SQL Server.
Fixed Database Roles exist for the convienence of assigning a pre-set definition of permissions to a SQL user account.
You can create a user account in SQL, ignore the Fixed Server AND Fixed Database roles, only assigning what permissions are necessary. Or you can create new Fixed Database roles for non-admin users that you want a mix of permissions attached to so you don't have to do this work several times over for every user you grant access to.
You can find more detail on these options in Books Online.
Catadmin - MCDBA, MCSA
Beware the error of pre-emptive poultry inventory!