If your business has at least three computers, you need a server.
Server-based networks are an essential ingredient of today's
business model, holding a number of key benefits over
peer-to-peer desktop networks, including:

Improved Security - Server based networks improve security by
restricting access to company data and files.
Improved performance - Server-based networks scale more
efficiently than peer-to-peer networks, allowing you to have
more employees working without losing performance.
Reliability - Mirroring the files saved on the server's hard drive
allows you to have multiple copies of company data. This helps
prevent the loss of information in the event of a hardware failure.
Centralized Storage - A server-based network allows you to
centralize storage of valuable company files and to use off-site
backups, adding another level of protection for your data.
Popular platforms for servers and workstations are made by
Hewlett Packard and IBM, as well as a few other companies.