It's the little surprises!
After every trip, when I'm in the processing phase of things, I discover little critters I had no idea were there.
Case in point - on this crab, there's a tiny juvenile Reticulated Brittle Star. The adults were all over the place, but I never would have seen this little one. And there above this xmas tree worm, there's a juvenile Nimble Spray Crab (I think).
To me, it's one of the real rewards of shooting macro.