The plants not only improve the aesthetic value of the tank and provide a sense of safety for the fish, but they also make a modest but discernible contribution to the cleanliness of the water in the tank as a whole. It is unavoidable for live aquarium plants to grow filthy, become coated in debris, and acquire a covering of algae while in the tank. That is a natural consequence of being in the tank. To put it another way, if you want to keep the tank in good condition, you must ensure that the plants are always kept tidy.