just like plah said, when fish pee, it creates ammonia, which is very, very deadly. but its not JUST ammonia, there are other minerals and substances in it as well. the ammonia is eaten up by bacteria already in the tank (in the filter sponge) and converted into nitrIte. nitrIte is also very harmful. there is another type of bacteria also present that eats the nitrIte and converts it into nitrAte. this is the least harmful. but it is also the end of the line. there is nowhere else the nitrAte can go. and if it builds up and builds up, along with all the other minerals and what we call "Total Dissolved Solids" in the water, it causes the water to actually get "thick" with all this stuff dissolved into it.
the ONLY way to remove these substances is through water changes every week. these substances do not evaporate, so just topping off the water that evaporates does nothing. water has to actually be removed, and replaced with conditioned tap water.
if your tank was a lake or stream, for one, there would be a much, MUCH smaller bioload, but the same bacteria would be present, PLUS there would also be plants and other microorganizms and bacteria all working together to eat up the extra nutrients. not to mention that the water would be seeping into the ground, and being replaced by rain water and snow melt, which is pure water, not containing any sort of dissolved minerals or waste.
therefore, in a closed system like an aquarium, without all those microorganizms and bacteria completeing the cycle, its up to us to play god and do water changes every week to keep the tank healthy.