Russia has always supported Assad, even back in the day when it was his father in power and Russia was part of the USSR. Russia is very concerned about Islamic militancy because there are a lot of Muslims in Russia who are part of separatist movements, especially in Chechnya. The Russians likely perceive Assad as being the only leader strong enough to maintain a secular state in Syria.
Because "everybody else is doing it".
Seriously, though ... Because they support the Syrian government, and Putin wants to regain the glory days of the USSR so is flexing his military might.
It's a fair question and the answer isn't so easy.
They claim to be helping in the fight against ISIS but the real reason is probably that Putin supports the Assad regime. In that role they'll bomb Syrian rebel targets as well as ISIS.