Vallisneria is often used as a background plant that grows to forms some surface cover. It does fairly quickly, not as fast as anacharis, and is a little 'neater' as it just grows straight like grass with no offshoots that a stem plant has. It almost creates a curtain along the back of the tank.
Aquariums are often very sterile, with a clean and inert substrate, plastic ornaments and plants, some dentrifying bacteria helping to detoxify fish waste, and a human armed with a syphon, dutifully keeping the place spotless. Its miles away from the home the fish have spent millennia evolving in. But, its a relatively safe environment; regular water changes remove nitrate, and gravel vacuuming, filter cleaning, lowers the risk of harbouring any dangerous pathogens.
There's different schools of thought with regards to the decomposition of organics such as fish waste, driftwood, dried leaves - almond, oak.. (not uneaten food from overfeeding). Some see the development of mulm from detritus as a means of creating dissolved organic matter (DOM) that adds beneficial carbon and humic substances (what makes soil good) to the substrate. Together with the assistance of organisms like snails, the substrate becomes an enriched mixture allowing for healthy plant growth. The microorganisms responsible for decomposition also create a lower level food chain (infusoria) perfect for bottom feeders such as corydoras and also fry.
Furthermore, the tannic acids released by some decomposing organics have proven antifungal properties.
The other thought process, is that this layer of dirt in your tank, is the perfect breeding ground for parasites and pathogens. Which is true. However its not the fault of the decomposed organics, its us (and fish farms) for introducing these nasty creatures to the tank. Its a shame. Because much like how our lives have now become, facemasks and hand gel, its probably better to keep the tank as clean as possible, even if this is unnatural for plants and fish.
Stress is widely known as the leading cause of many illness in fish. When a fish doesn't feel safe, because it doesn't recognise it's surroundings as natural (neon glittery pink gravel or a mismatched stocking of community fish), it becomes stressed. This compromises their immune systems and makes them susceptible to the pathogens living in the water. Much like how we are constantly surrounded by viruses and bacteria, but our immune system is keeping them at bay.
A well planted tank provides them with a stress free environment. Some plants also release allelochemicals that work to inhibit growth of some algae such as cyanobacteria. (Cyanobacteria releases cyanotoxins which are dangerous to fish).
All in all I believe there's a strong relationship between fish and plants, and its a well known fact that plants love dirt.
Keeping everything pristine clean, buying filtration, water treatments, and stocking up on aquatic medicines should the fish succumb to an illness (because they're stressed...), certainly keeps the pockets of the fish keeping businesses topped up.
Anyway, sorry for the waffle!
If theres a chance of overfeeding, and in a bid to help keep those nitrates low, I'd recommend adding some type of snail if you haven't already. They're great at indicating whether you're overfeeding, as their population will explode from eating the excess food.
And yes Vallis, I recommend some Vallis!