Here are some of the benefits provided by plants:
While HOB (hang on back) filters and canister filters provide good mechanical and biological filtration, plants provide a unique kind of filtration. Plants can absorb and remove all types of waste created by fish, excess food, decaying materials, and even heavy metals. In addition to this filtration, plants also create additional surface area for beneficial bacteria to grow on. This provides additional biological filtration. Mechanical filters can lose effectiveness over time if not cleaned properly, but plants will continue to act as good filters as long as they're growing. The overall quality of water in aquariums is higher if plants are growing in it.
Instead of placing airstones and air pumps into aquariums which aren't aesthetically pleasing, plants can provide adequate amounts of oxygen in aquariums while absorbing carbon dioxide released by livestock. Plants can do this because they perform photosynthesis. During this process, plants absorb nutrients, light, and carbon dioxide to release oxygen as a by-product.
Algae growth is a common problem in aquariums and difficult to deal with without plants. Algae grow due to excess nutrients and light in aquariums. To combat the growth of algae, plants can be used to out-compete algae for nutrients. The more plants there are in an aquarium, the less likely you will see algae growing in that aquarium. In fact, if an aquarium is in balance, where the plants are growing well, very little maintenance is required.
Sometimes certain species of fish can be very territorial. Plants allow different species of livestock to coexist by providing cover and protection. Some types of fish even breed and lay eggs on the leaves of the plants. With the presence of plants, the ratio of the number of fish per gallon is greatly increased.
With all of these benefits provided by plants in aquariums, it doesn't make sense to have a freshwater aquarium without plants. Think of it this way, an aquarium without plants is like a world without trees. More information on how to keep plants happy and growing in another blog post. Until then, cheers!