There's something nice about the idea of other artists creating something that is in some way inspired/based on another piece of work. It's also interesting to see how different the result that artist x produces is from the result that artist y produces.
Oh cmon, the real drive here is ego, let`s face it. Artist X gives Artists Y parts for a remix. Artists Y thinks, "yeah, I`m gonna kill this" and just mashes it all up and makes his own tune. There`s very little "inspiration in 70% of the remixes I hear".

The other motivator, is getting a bigger name remix for a label, in the hope that it raises the profile of the label, which is just kak really. Hijacking a piece of someone elses name.

Oh and then there`s the, "well let`s give the new kid a chance, but I don`t wanna get overshadowed, so I`ll make him do a remix, that way, it`s still a bit of me, rather than an original tune, cos he might do too good"

Call me pessimistic, but I`m sure there`s some truth in what I say.