Russian jets in Syrian skies
Like Putin or not, he's only in this for Russian interests and not out of the goodness of his heart. Much in the same way that the UK, US and various other nations are involved - national interests of one form or another.
The obvious reason for Russian involvement is that Russia wants to secure it's Mediterranean access through the port at Tartus. However, there are other economic and geopolitical factors at play with oil and gas a key driver.
Gazprom signed a deal to market Israeli liquified natural gas earlier in the year as well as having it's eye on a long term strategic and economic position in the eastern Med, most likely by tapping into the Eastern Mediterranean Levant basin's holdings, estimated by some at 1.7bn barrels of oil and 120 trillion cubic feet of gas. Given dwindling efficiency in domestic energy production, competition from US shale and western European nations wanting to look elswhere to reduce the threat from Russian dominance, Russia's economic future now lies in being able to secure influence over and access to oil and gas fields and secure methods of production and supply. And this is where a compliant state within its southern sphere of influence and with access to the Med comes in handy.
Politically there is also an imperative. With a large number of ISIL's toughest fighters coming from Chechnya, he wants to prevent them returning to foment even more problems in the Caucuses. As such, don't expect his tactics to be particularly restrained; if it means wiping out a few towns and villages or bombing the odd mosque, I'd wager he'd happily do it if he thinks it removes the threat. Cue what western liberals might view as less than precision or discriminate bombing.