"I'm something of an enthusiast... but what do you get out of a history class that you don't get out of a history book?"
Some people (myself among them) prefer to learn by initially getting taught the subject basics from a person that knows the subject matter. This allows you to get direct feedback to your questions, gives you an idea of the overall scope of the subject and sets you thinking on the right path.
For example, I find trying to learn a new programming language from scratch from a book very challenging. Whereas if I attended a basic course on it (even for a few hours or days just to cover the basics), I learn it much better and faster. I then go off and buy the necessary books/search the web to augment my knowledge. Different people learn differently. I just happen to prefer human contact at the outset, whereas others will pick you a book and teach themselves to become experts all on their own.
1) A good idea about which history books to read, and in what order.
2) A collection of knowledgeable colleagues with whom to discuss history
3) (optional) A pretty certificate.
Any professor worth their salt will guide a debate among the classmates, and try to get students to argue about the interpretations of events, instead of spoonfeeding them from a history book.
So, what do you get? A set of different views coming from your classmates. An exposure to methods of debate. And so on.
(if the economy was worse when I came of age, or if my family was wealthier, I probably would be some sort of history-related academic today.)