The reason that Russia is sending aid to Syria is very simple: While the USA has around 2200 military bases around the world, Russia has exactly one and that one, Targus in Syria, is critical to their naval forces. The reason is that the only Russian warm-water fleet is the Black Sea Fleet. But the BSF's home is at the northern side of the Black Sea. Turkey, a traditional enemy of Russia and nuclear-armed NATO member, completely controls entry and exit from the Black Sea. Turkey could easily deny the BSF access to the Mediterranean Sea and the rest of the world by blocking, mining, or bombardment of the Bosphorus and Dardenelles. This would essentially bottle up the BSF in the Black Sea in times of tensions.
The base in Syria is important to Russia because it would allow them to refit, refuel, and rearm the BSF without having to transit Turkish straits into the Black Sea. This is the specific reasons for giving Syria sophisticated anti-ship (and especially anti-carrier) missiles. But it also allows Syria to exercise some degree of control of the Mediterranean from Syria to south of Israel. This would allow them to attack naval forces off both Lebanon and Israel, a serious escalation of the regional problems.