Why should we care about what's happening in the planted aquarium substrate?
I think most of us intuitively know that the substrate microbiome is one of the least studied topics in the planted tank hobby. Many find it irrelevant as it seems to not be a determining factor for creating a healthy and vibrant aquarium. Others accept that a substrate microbiome develops over time in the aquarium and may contribute to the health of the plants. Again, a shrug of shoulders because, unlike adding CO2, it doesn't seem to make much of a difference.
While I understand the above points, I believe that some hobbyists enjoy the science associated with our hobby and want to push forward our understanding of the living ecosystem we love to create. It's not an inanimate piece of art or a piece of furniture, we are all creating an environment for many forms of life and that responsibility requires that we try to understand and improve our knowledge of how to create the best environment we can.
At the moment, we have little to know understanding of the relationship that the microbiome that develops in our substrates has with aquarium plants. We do know that this relationship has existed for eons and our only now learning how extensive it is. Could a plant that mysteriously isn't doing well in your tank be missing something like this? Could a healthy microbiome lead to stronger, healthier plants that better show color, fight of algae and grow?
Take a listen to this Ted Talk when you have a little time. It's on the relationship of mycorrhizal fungi and trees but try to think about how it would apply to our planted aquariums.
I previously linked a resource related to a study finding mycorrhizae in aquatic environments too. Here is another one talking about Lobelia and mycorrhizae.
Businesses have been and are expanding their focus on the substrate microbiome

Anecdotally, I built a few ADA only aquariums in the past. I have yet to have an easier time with any of my other tanks. The ADA tanks that contained Bacter 100 and the other products, just ran and grew better.Bacter 100 is a substrate additive, containing more than 100 kinds of substrate bacteria in a dormant state. By sprinkling on the base substrate, it can make an ideal substrate environment. By applying on top of blue green algae, it suppresses the growth of it.
Snake oil? Is my experience, simply my experience and can be attributed to many things?
I think in today's science, we know that monocultures of bacteria are bad so more bacterial species in harmony is better. We also know that bacteria and other microorganisms do a lot more in symbiosis than we first thought. We are learning this even about our own gut biome. Why do you think this wouldn't apply to our planted aquarium environments?

Even CaribSea, a company that focuses on substrates, has quietly gotten into this with their product, Flora-Spore. I've reached out to them to get some background on this product but, alas, my requests have gone unanswered.
According to their product labeling:
Flora-Spore is a mixed blend of mycorrhizal species specially created to help create a thriving and dense root system in plants while increasing their ability to naturally extract elements directly from their substrate, allowing for better uptake of minerals from fertilizers. Mycorrhizal fungi have traditionally aided plants in widespread growth by acting as the intermediary between their roots and solid minerals. In the natural world, these organisms form a symbiotic relationship with most all vascular plants, making them a central component of the rhizosphere (the ecological zone existing around and influenced by roots and associated micro-organisms). Once they have colonized a root system successfully, mycorrhizal fungi secrete acids to dissolve surrounding minerals, which aids in increasing the efficiency of absorption of essential elements to the host plant. Recreate this natural (and hundreds of millions of years old) relationship in your habitat with CaribSea's Flora-Spore!
I suspect that this product is similar to the TNC MycorrHydro produced for the hydroponics industry. Now I'm sure it's not the same but I think the intended purpose is similar. TNC says is also has hormones:
TNC MycorHydro is a highly concentrated water dispersible mycorrhizal inoculant developed for hydroponic applications. It contains 15 different species of mycorrhizal fungi, including Endomycorrhizae and Ectomycorrhizae as well as 5 species of Trichoderma. It also contains 13 Bacillus species – Bacteria for Nitrogen fixing, Phosphorous solublising and growth promotion.
TNC MycorHydro also contains natural bio-stimulants such as humic acid and derivatives of a natural marine algae (Ascophyllum nodosum). These include amino-acids, plant hormones – cytokinins, auxins and giberellins – as well as vitamins and trace elements.
Is there an easy way to get some of the benefits without commercial products?
Think about this. What if I went over to @GreggZ's house and he was kind enough to share one cup of his substrate to me? I then came home and added that cup to my substrate. I've just taken a good sample of Gregg's microbiome and added it to mine simply adding to the diversity. However, as Gregg's tank is clearly successful, I'm willing to be that the microbiome that has developed in his tank is one that works well for our purposes. Why not "bottle the magic!"?Of course, this doesn't mean my tank will now be like Gregg's. This isn't DNA cloning. However, it does certainly function like me taking some probiotics. I'm adding bacteria, fungi, etc. that are beneficial.
Let's extend this. Why wouldn't Marian Sterian or Dennis Wong not package up cups of their successful tank substrates and sell it to others? Wouldn't the same apply?