Setting a tank up with marine plants is not an absolut necessity for a sucessfull tank. However, they can be quite usefull and enjoyable as well. In short, plants will utilize fish waste (ammonia and phosphates) as a food source and thereby help lower those levels in your aquarium. During the light period they also utilize CO2 and give off O2. Some people will take advantage of that and remote plumb a tank of plants and light opposite to the main tank to help stabilize pH fluctions that occur day to night.