Have you played Baldur's Gate or similar games before? Combat is what the game is about. There's a lot of other cool stuff but that's the main thing.
As for combat builds, are there any necromancer/summoner abilities, enough to warrant investment?.
Yep, and tons of different ways to do it. Oddly enough, Necromancers are not the best at summoning though it can be done. Necromancers excel at "save or be fucked" builds so they typically invest heavily into making saving against their spells difficult.
However summons are nice and plenty of classes can do it well.