What are the advantages of adding a Mac to Active Directory?

One of the main reasons you would do it is to authenticate users against Active Directory. By authenticating against AD it also makes access to file shares and printers much easier, as you are already authenticated.

It helps with asset management. Most asset management pieces that plug into AD can query for a list of computer objects. As long as those are kept up-to-date, you can use that to help track systems in the organization.