You need a server if:

• You need to share a lot of data among your co-workers.
• You need to share data among your co-workers, and your internet connection is not that reliable to store the data on a remote hosting.
• You need to share confidential data among your co-workers.
• You would like to provide some special, non-standard services via the internet.
• You need an internal email/web/torrent/VoIP/storage/etc server, which can't be located outside the office for security (or any other) reasons.

Ready to buy a server? Stop! You can rent a server if:

• You think that a server in your office will increase productivity, but not 100% sure to pay a huge amount of money for it.
• You would like to buy a server, but you're afraid that the service will fix it too long when it's broken.
• You need a server for a short period of time, so no sense of buying one.