My guess is that he wants our soul because the tadpole might eventually turn us into something that's going to be useful for him to have control over. Whether that is going to be a powerful mindflayer or some other abomination, I have no idea.
But I guess he wants it for the same reason as every other powerful being in the game wants to get to you. I just hope that (eventually) accepting his deal would give you some sort of boon that's unique to him or something.
