A H.323 gatekeeper controls all H.323 clients (endpoints like MS Netmeeting) in his zone.
Its most important function is address translation between symbolic alias addresses and IP addresses. This way you can call "jan" instead of knowing which IP address he currently works on.
Why should I use a gatekeeper ?
If you currently use a client and are happy with the way it works there may be no reason to use a gatekeeper.
If you however work in a larger VoIP setup using symbolic addresses is definitely a great plus. More importantly the gatekeeper is the key to the management of a VoIP network.
VoIP gateways for example register at the gatekeeper and the gatekeeper finds the right one for you to use to call a specific number.
A gatekeeper can also manage the bandwidth used by the clients.