Often one computer will be used to fulfill multiple roles. Click on one for more information. They are useful for managing large numbers of users spread over a large number of machines. If this is all a machine does, then a Pentium-75 should be good enough. Drive size and speed are not usually limiting factors.
Network connection is a limiting factor.
File Servers File servers provide a repository for data. Individual uses or groups can set up directories that are accessible from anywhere in the world or down to a single computer. Security can be based on user, location, or both. A backup drive attached to such a system provides data integrity in case of power failure or accidental deletion of a file. File space and disk speed are usually the limiting factors here.