The reason why Germans wanted to advance there was to deal a significant blow to Western Allies, which could serve as ground for a potential truce, as opposed to eventual complete capitulation.
Same here with ISIS. They probably want to strike a deal with one of the major sides so they could at least have a chance of surviving. They also know that the times is against them: as soon as Trump is in the office, he'd probably dedicate much more effort to deal with them.