The water from your tap is supposed to have about 2 ppm of what is called chlorine residual. That's the chlorine that is left over after the chlorine has killed all of the bugs in your water. That way, there is some chlorine left over to kill the bugs in your water tank.
The problem with chlorine, is that it dissipates over time. Your tank won't be completely empty and refilling with fresh chlorinated water kills any bugs that might grow during your home time.
It removes a small risk, and I find that in hot weather, the water starts to taste like plastic after a bit.
2009 Roo 21ss + 2007 Superduty 6.0