Well, Raphael, just like any devil, has only motive: makes you and your party souls his so they can be traded or used in the blood war.
I heard about this blood war before. I think the tiefling that curses a lot mentioned it. What is it exactly and how much one soul worth?
When your soul is taken by a devil, it is reborn as a lesser devil or transformed into a soul coin, like the one you get from the tiefling. As a lesser devil, you fight the blood war against demons on the river styx. As a coin, you can be used by inhabitants of the nine hells as a currency.
Either way, a soul is a valuable asset for devils. And the pacts are used essentially to bribe a creature into giving up their soul after they have enjoyed the gifts offered by their infernal benefactor.
