First, no demon or succubus is usually looking as in these sexy pictures you posted. They don’t even appear in dreams like this… at least, none (and I had many visiting) ever appeared like in these succubus drawings for me.
One once showed me her true form in a vision and from another I saw it in the astral, but they are different from how people portray them, in these sexualized and other everything-is-cute-these-days images…
What they want in exchange? Well, some would say, your SOUL 
! No, actually a lot would say that and believe so. I don’t know if that is true, but they surely are looking for energy. If other reasons exist, why a succubus may be interested in you, energy may be not the main “trade” for sex, but usually it will be.
But, before you progress, check out this thread:
From personal experience I can agree with many of these points and fyi, a succubus (or demon) lover does not mean “easier”.