They are simply there to keep their ally Syria from falling. The Russians are not good at strategy in advancing their interests. In this case, let them fight ISIS to their loss if they lose, and the West’s gain if they win, with one caveat, this will not end well for them either way. Let them spend their resources, not ours, but then again, this could be only a temporary measure, they will leave when their losses mount, they are already in poor economic straights. See Ezekiel and Revelation.
It's chilling. Who would have thought, ten years ago, that Russian combat troops would be in Syria? No one. Yet there they are, soon to be joined (if reports are true) by Chinese combat troops.