One reason the Russians have combat troops in Syria is because this gives Russia a chance to put its post-Cold War military to the test. What the Russians are preparing for is the possibility of clashes between Russian and NATO forces in Eastern Europe. Both NATO and Russia are not sure how their respective post-Cold War forces would do against each other. Most East European nations are preparing for the worst and paying close attention to whatever Russia does in Ukraine and Syria.
The most likely targets for Russian invasion are three small nations (Lithuania, Latvia and Estonia) on the south coast of the Baltic Sea between Russia and Poland. In the 18th century the Baltic States were forcibly incorporated into the expanding Russian empire. They became independent after World War I (1914-18) but were taken over again in 1940. It wasn't until 1991 that the Baltic States regained their independence and they are determined to keep things that way.
The Baltic States have only 6.3 million people and fewer than 60,000 active duty troops. Well aware of their vulnerability the three Baltic States along with neighboring Poland joined NATO in 2004. This was done in the hope that the mutual defense terms of the NATO alliance would dissuade Russia. It did that, but it also angered many Russians. Government leaders there, like Vladimir Putin, considered it an act of NATO aggression.
All three Baltic States are rapidly upgrading their armed forces and building a reserve army (like Switzerland, Sweden and Israel use) so that in the event of a Russian invasion (or threat of one) enough armed and trained personnel would be deployed to make Russia think twice about going in. The modernization and build up is also considered “aggressive” by the Russians because given the forces available to Russia and NATO 50,000 or more trained and organized reservists in the Baltic States makes a big difference.
In effect, now that Russia has threatened the Baltic States enough to trigger a modernization and expansion of Baltic States forces and for NATO to revise its joint defense plan for the Baltic States, the chances of Russian success are declining. That is why Syria is so important to Russia. The Russian problem is that while they, and all other European nations greatly reduced their armed forces after the Cold War ended in 1991, Russian forces were hurt most of all. The forces of the now defunct Soviet Union were, by the end of the 1990s, were reduced to 20 percent of their Cold War size. Worse, very little new equipment was purchased for about 15 years after 1991. And a lot of the Cold War era weapons and equipment were questionable even when new.
Not only had Russian forces shrank but they had less training (no money for it) and less capable officers (the best ones left for more lucrative and fulfilling civilian jobs). Since 2005 Russia has been trying to modernize its forces while also providing adequate training and better leadership. It is questionable if the Russians have succeeded. The ground forces can only muster about 55 brigades, compared to 175 divisions (each the equivalent of about two current brigades) and over a hundred reserve divisions in 1990. The Russian reserves disappeared in the 1990s, along with their weapons and equipment. A new, smaller reserve force is now being developed.
Military simulations (wargames) of a Russian invasion of the Baltic States indicates that if everything went in Russia’s favor the Russian troops would overrun the three Baltic States in two or three days. This assumes that NATO only gets about a week’s warning that the Russians are massing forces (20-25 brigades and several hundred aircraft) on the borders of the Baltic States. NATO already has about a dozen infantry brigades ready to be rapidly (by air) moved to the Baltic States in an emergency. Heavier brigades, with tanks and other armored vehicles would take longer to reach the area. The Russians would seek to occupy the Baltic States, and defeat twenty or so combat brigades of the Baltic States and NATO within a few days. This would require Russian air power to be capable of neutralizing NATO air power for a few days. That is a major unknown and one reason Russia has several dozen of its newest warplanes in Syria operating under wartime conditions. But it is still unclear if Russian aircraft and anti-aircraft systems could defeat NATO air power. Russia is also testing new artillery, other weapons, communications and electronic countermeasures gear. All would be used for a go at the Baltics and Russian or NATO simulations of such an attack are much more accurate if you know how new Russian equipment and forces perform under fire.