A small list of things that are possible:
Throttle bandwidth for certain computers
Throttle bandwidth TO certain computers
Help you to fairly share your bandwidth
Protect your network from DoS attacks
Protect the Internet from your customers
Multiplex several servers as one, for load balancing or enhanced availability
Restrict access to your computers
Limit access of your users to other hosts
Do routing based on user id (yes!), MAC address, source IP address, port, type of service, time of day or content
Currently, not many people are using these advanced features. This is for several reasons. While the provided documentation is verbose, it is not very hands-on. Traffic control is almost undocumented.