It seems that it is an routine after-school endeavour for the Gremory to be summoned through their flyers to do devilish stuff like foot massages and claim the dead as their pawns.
Aside from the summon shown in Ep. 1 (when Rias is summoned to claim her Pawn) and the paiting Azazel gives Issei, there are not shown in the anime what rewards the demons gain. It is mentioned towards the end of the second season that Gaspar makes the most out of contracts than any of them.
What do they get from these demonic contracts? And how exactly does fulfilling contracts help a demon rank up?
Side note: I have only watched the anime, but answers are encouraged to come from all media.