Why is Russian Roulette used in ray tracing? Why should we use Russian Roulette? Does it reduce ray tracing artifacts?
The most usual way of employing Russian roulette in path tracing is to cut paths with high attenuation (low throughput) without adding bias into the resulting estimator, which would occur if you just use constant maximum path length. 

Although Russian roulette incorporates additional variance into the resulting estimator, it helps throwing away long paths consuming a lot of processing power while contributing little into the resulting image, possibly leading to increased performance of a renderer.