A common trait of deities and demons is that they are immortal. A downside to immortality - there's nothing to push you to develop new ideas, art, or technology. While a demon may not need such ideas, they may crave novelty but be unable to generate it themselves. Thus they set themselves up as patrons of arts and science, drinking in the new ideas of these brief and changeable mortals like wine.
Being immortal, these beings would have no problem setting in motion plans and schemes that play out over generations of human lives. They might value a mortal doing some apparently insignificant service for them, which will contribute to some grand scheme that has been building for generations.
This is more or less an expansion on their ideas, but a supernatural immortal entity might well find that conversation with a mortal shows them new ways of looking at things that they would never have come up with by themselves.
It's a common trope in fantasy that some supernatural beings only exist and only have power if they're believed in. A demon or deity could well chose to grant boons to mortals simply as evidence of their existence and their power, to sway belief in their direction and grant them greater power and prestige.
This is the great power source of creation, slightly easier to discover than fire, only slightly harder to discover than water. The Ds of course know about it, and mortals are just full of it, but the Ds can't drain it if the mortals find out about it and start using it themselves. Thus, they keep us busy and distracted with baubles like kingdoms and empires and antibiotics and stardrives to keep us from discovering slood.