Basically he fulfills one of three roles depending on the matchup. What makes him amazing is you can adapt those roles on the fly, and never have to pick in advance.
Turn 1: If after your hero phase comet/realmshaper/etc they have multiple support heroes on 1-2 wounds left, bring him down and shoot them. Then he just is a mobile artillery piece that can fight for the rest of the game. (Lumineth, Nighthaunt, Grots, Seraphon)
Turn 2: When I usually bring him down, great counter punch, auto-12 inch charge, 5 attacks, and still plenty of time for shooting.
Turn 3: If you REALLY need him to punch hard in combat, you lose most of the benefit of the shooting, but 7 attacks hits like a truck. This is when I bring him down against armies like Archaeon and friends, some stormcast builds, maybe nurgle. Basically armies that the shooting wont hit more than 2 units, and dont have a huge alpha strike you need to try and pre-empt