All the reasons you can imagine. They like humans, they hate other devils, they get benefits... It makes them stronger, or it amuses them.

[ Blinking, Aki turns on the ocular before he starts to keep moving, using his metal hand to follow along the walls, just in case there's any kind of loose nails or possible damage. Better to scrape metal than flesh, as far as he's concerned. ]

I've never given Kon anything that didn't grow back. Other devils are more particular.