Besides introducing 11 new base classes, the PHB II focused primarily on helping players better define their characters. It gives suggestions of back story typologies, helps define character archrypes (personality vs. Class defined), introduces the character traits one can pick at level 1, for feature variation.... Basically, it provides a lot of methods that could help a player who always likes playing a Barbarians (or any other class for that matter) come up with completely unique characters each time, instead of just playing Steve, or Steve's identical Twin, or Steve's first cousin....

In addition to PHB II, the book Unearthed Arcana also provided some incredible class and character variations. That said... Neither of those two books spend much time on the rules of the game. So if you were ever wanting a book that can be used as a resource for new players... The core Players Handbook is the way to go.