They help use up nitrates, which is the last part of the cycle that adrinal was telling you about. Nitrates are harmful for fish in high doses, so plants should help you out with that (as should regular water changes). You shouldn't add plants just to remove those nitrates by the way. People are told not to add catfish to clean up the tank, so the same goes for plants.
Anyways, they're beneficial in removing nitrates, they add beauty to the tank, some of them reproduce quickly and readily. They don't really add that much O2 to the tank. Research shows that they do almost nothing to add O2 to the water (agitating the water surface does this).