They do take nitrates and ammonium, but not enough to really make any kind of change. They are very, very slow growers, and are somewhat infamous for having other types of algae grow on it if water perimeters aren't kept in check.
Basically they're alive, green, and fish like to sleep on them. Bio-film eaters like cherry shrimp and some snails will graze on the things that grow on them, but as far as I know none of the common aquariums creatures actually eat them.
You can rip them apart and tie/glue them to driftwood and rocks and they'll attach after some months and actually look quite nice.
They're actually good beginner plants because of how simple they are to take care of. It's basically like having the aquatic form of a pet rock really.