Most water heaters I've seen are held in place by their own weight and, perhaps, the water connections. In earthquake prone areas that's not enough. Think of shaking a table with a tall glass of water sitting on it, or check out these movies of simulated earthquakes:http://seismo.berkeley.edu/~rallen/seishaz/buildings/CUREEtest/http://seismo.berkeley.edu/~rallen/seishaz/buildings/CUREEtest/officedinning.mpeg
See how the water heater just falls over?
You might be thinking that if there's a strong enough earthquake to tip over my water heater, I've probably got bigger issues. Perhaps true, but I don't want to add fire from broken gas lines and flood from broken water lines into the mix.
You'll note in the pictures, I've already installed a flexible gas line, and semi-flexible water lines. The water lines are ribbed copper pipes that have some flex to them.