It helps combat algae, I suppose. It will also allow you to do a "silent cycle." This is when you use the plants, which use up nitrates and ammonia in your tank water, to help keep down the bioload in your tank. The plants use up the wastes before they are able to hurt the livestock. It is generally advised to do this with fast growing plants, though. Stem plants in general are good for the job and anacharis/elodea/egeria densa is a common stem plant that most people like.