They have a good rapport, listening to them ramble a load of old nonsense is a lovely way of spending an hour.
As Enzo says, if all Andy did was laugh at Bob's flights of fancy, his role would be redundant. But I think he does a good job of joining in while remembering to sit back when Bob is on a roll. Without him, it would just be Bob wittering away to himself, which wouldn't really work. He needs a foil, someone to bounce off.
Bob is obviously one of the funniest men in the world, I think I'd be too in awe if I somehow found myself co-hosting a podcast with him. Andy clearly adores him, but he's not afraid of taking the piss. The balance of power is roughly equal, they both needle each other in a good-natured way.