Primarily, they help maintain optimum water conditions by absorbing carbon dioxide and, to some degree, remove nitrates from the water.
In addition, they provide fish with shelter, spawning sites and, in some cases, food!

In the aquarium, you have the choice of using real living plants or equally realistic plastic replicas; either will beautify the aquarium and whilst the plastic ones won't contribute any water-cleansing functions, they still make the aquarium look nice if you like to keep fish that have strong vegetarian-diet tendencies!

Live aquatic plants are usually classified by hobbyists into three groups:

                                   Rooted, Floating and 'Cuttings'

Rooted : these plants generally feed through their leaves but have roots for
             merely anchorage purposes. Another way of anchoring themselves in
             position is by clinging on to rocks or logs with thin creeping stems.

             Most rooted plants send out vegetative runners on which young plants
             grow; alternatively, some have 'daughter plants' growing on leaf
             surfaces.

             Members of the Aponogeton genus bear aerial flowers which, when
             pollinated, produce seeds from which extra plants can be grown.

Single, large specimens of rooted plants are best used as 'feature plants' in aquascaping designs, whilst the tall, grassy species provide natural 'curtains' which hide the back and side walls of the tank.

Floating : As the name suggests, these plants are happy to float around
               untethered. Some have nutrient- collecting, trailing roots dangling down
               into the water and these make excellent refuges for young fish.
               Surface floating plants often become rampant and should be netted off
               as required.

Floating plants offer shade, shelter and, sometimes, bubblenest building material for fishes.

'Cuttings' : Fast growing, bushy plants can be propagated by taking cuttings and
                 using these to make more plants by re-anchoring them in the
                 substrate. Fine-leaved species such as Cabomba and Myriophyllum are
                 examples of such plants.

These plants provide an excellent and natural way to fill up corners and spaces when aquascaping the aquarium. Dense bunches of such plants also make excellent spawning materials for egg-scattering fish.

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