For me:
-Cash savings due to lower cost independent A&P vs the higher dollar bigger shop with overhead costs. As long as he knows what he's doing...
-I can prep the plane for service (inspection panels, cowlings, wheel pants, seats, rear bulkhead, etc.) even before he arrives and I can reassemble as needed/allowed under his supervision. Saves time and thus dollars.
-Education. I get it as part of my payment. Hands-on, stupid questions, observation. Adds time though. But that's worth cash to me, if I compared A&P labor rates of equal cost, one allowing owner assist, the other not allowing me in the shop. I want to know the systems of the plane inside and out, and see and put fingers in the guts of my bird.