My guess is that one or more of several reasons prompted the Russkies to do this.
1. An oil pipeline from the non-Russian Caucasus oilfields goes through Georgia, specifically to bypass Russia, and lessen Russian control over oil production and distribution.
2. Georgia has been making attempts to join NATO, something Russia doesn't want on its borders.
3. Russia is flexing its petrodollar-fueled might by demonstrating its army is still a viable threat. Russians have a proud imperial history, both during the Czarist days, and during the Soviet times. I can't imagine they like the loss of standing their country's experienced over the past 15 years or so. And nothing brings a country together like invading someone else (unless you're the US, in which case it tears you apart).
4. Russia likes having satellite puppet regimes on its borders as a kind of buffer state. They basically dictated policy to the Georgians for centuries, and see it as their right.