Yes, they do. They share a border. They have bases in the area. Yes they had a deals. You know what? So does the USA have basing rights in numerous countries.Err..... like?
Oh, you mean because Assad is Putin's client, crony and butt-boy in the Arab World, so Putin has to send in troops to prop him up? Gotcha.
Beyond that here are two other reasons.
The fact that several IS commanders are Chechen. So his taking them out sends a message back there. And killing their success in Syria stops them from feeling emboldened and starting up at home.
He has also been a using the Russian Orthodox Church a lot to. And their are historical ties to the Orthodox Christians in Syria. The original church for all the Orthodox offshoots is thought to be in Damascus. Nice excuse to make points with that while completing whatever real agenda he has.
I'm no fan and apologist for Putin. And I know how you feel about him. But the sources for this came from my various readings including the G And M, NP, Star and some NY papers. Take the blinders off and see how this has a direct effect on the Russian domestic front, and realize he does have a legit stake.