babyfish
Fishaholic
Some plants like hornwort will pump anti algae chemicals into the water.
Your fish will be seen more often if they have somewhere to go and hide.
Plants will add a SMALL amount of oxygen to the water, in an under populated tank this is usually sufficient for the fish's needs.
Plants will reduce nitrate and help to soften the water as an aquarium ages.
They just LOOK GOOD
you want some really low-light plants then...try cryptocorynes, anubias, java fern or java moss, or any others that you can find on tropica.com that are labelled "low light"