What did they do with his character that they didn't already tell in the movies/novels?
They go far into his past, to his childhood and tell about that. Going to college. When he first became his new self, not just showing how he reformed himself, but other stuff as well. To Mars when he left/"abandoned" Earth during Watchmen. It's heavy on science.
The main focus has actually been on explaining why Dr Manhattan doesn't fully use his powers all the time.
Dr M is shown to change reality with his powers. When he does that, he sort of breaks reality. It starts to have an infinite number of parallel realities, basically following both paths of each time Dr M has to make a decision in his life. Finally Dr M obtains an understanding that when he rewrites reality, he does not overwrite reality. The original 'what was supposed to happen' still happens; he just creates another world where he can have it his way instead. So in order to prevent all of reality from fracturing to splinters, he has to kind of stand back like an observer and allow a lot of things to happen, instead of directly controlling everybody.
It answers questions like how he can be able to halfway see the future, but why he doesn't explode babies in their crib based on their future actions. As he steps back and becomes an observer of mankind instead of a participant, he eventually loses touch with the rest of humanity. All powerful and yet totally helpless.
It was my favorite of the Before Watchmen series (I'm also following Ozymandias and Rorschach, and dropped Minutemen and Nite-Owl after the first issues). In it he becomes the Quantum Observer, collapsing the infinite possible paths of his life (essentially wave functions here), into a single reality. If you're at all into Quantum Physics (I'm twenty-seven and have been studying such since reading Hawking's Brief History of Time back in forth grade), no other comic book explores the matter so deeply or entertainingly.