question of the evening...

I've seen something in a couple of different databases that kind of confuses me.  I've been looking through books on line to see if there's any advantage to it, but haven't been able to find anything...

What I'm seeing is multi primary key's in one table.  I can't really think of what the benefit of this would be...isn't the purpose of a primary key more or less to provide a unique identity for each record?  If that's the case, why would you ever need 2 unique identity's?

Has anyone else seen this done before?