Tap water can have all sorts of nasty stuff in it that you don't want in your tank. Chlorine, chloramine, pesticides, phosphate, nitrate, ect... An RO
unit will remove all this and leave pure water. The effect of some of these is cumulative...water evaporates from the tank leaving all the nasties behind. This is the biggest reason that water changes are recommended. Every time you do a water change, you are diluting the pollution in the tank. Using RO
water cuts down on the pollution as well. Keeping nutrient levels down will help with the undesirable algae. And, yes, the bad algae will take over if you let it.