Some people do this monthly. Like jb said, some of the plastic valves can break and leak(or like mine, the nipple cracked and only leaked occasionally on the inside of the outer tank cover). You can put in a brass nipple, ball valve, and hose adapter to drain them easier. The TDS (total dissolved solids) and minerals in your water can build up on the bottom. Some plumbers rig up a spray wand and use a wet/dry vacuum to help get all the crud out of the bottom. When you get a layer of it on the bottom, upon heating, air pockets form underneath the layer and then break through up above. That makes the loud popping and cracking noise you hear when some water heaters heat up.
ABSOLUTELY DO NOT have electricity going to the heating elements of an electric hot water heater without them being covered with water or the elements will burn out. YOU MUST kick the circuit breaker off on the circuit going to the water heater before draining and have it filled before reenergizing the circuit by kicking the breaker back on.