AD gives you access to many more features than a workgroup can ever offer, such as domain authentication, security, a easy interface to administer user accounts, printers, groups,configurable OU's, distrobution groups and many more options including the ability to manage your users desktops and give them local rights on the pc's or manditory profiles so they can't change settings on the local pc.
There is a lot more you can do with AD these are only a few examples.
