People like gravel as it looks nicer than having plants just in randomly placed pots.
The 'gunk' in the gravel is what many of the plants feed off. A certain amount is fine and good.
The amount of gunk depends on how many, and what size, fish you have and how often you vacuum it. I clean out my heavily planted tank with no problems. I thoroughly clean the open gravel but around the plants I lightly hold the vac above the plants, or gently push it over the plant to rest lightly on the gravel, to remove the surface gunk only, leaving the rest for the plants.
All my plants are in aquarium gravel and I do not use any fertilisers or additives.