All living plants need fertalizer of some sort. In the wild it comes from decomposing matter, in the aquarium we need to add it ( generaly but not entierly true ). N,P and K plus micro, macro and minerals are needed.
In a very basic low light tank, fish waste can provide all the elements needed. But for tanks with high growth more fertalizers are needed, this will keep plant growth at its best and algea to a minimum, providing you check on Co2 levels etc.
Graeme.