IMHO basically we know very little. One thing we do know is that they are not only critical for breaking down old biomass but are 100% non-negotiable partners for many specific angiosperms. Once you lose the soil biome, those species just don't grow. Therefore when transplanting something it can be handy to bundle a sample of nearby earth at an appropriate range of depths. This can sometimes enhance germination and growth potential for difficult species by including a fungal colony... especially in difficult biomes like denuded or pool soils where a critical fungal symbiote is generally more likely.

This technique was taught to me by propagating native plant scientists here in Australia, which has a great number of unique plant species owing to its historic isolation and consequent evolutionary independence.

We know so little. We perceive and conceive things only to the extent that our minds and senses allow us to. Maybe there’s much more to reality than we can comprehend, and our current human capabilities simply aren’t advanced enough to perceive it yet. A few hundred years ago, we didn’t even know that microbes existed or that the light spectrum extended beyond what our eyes could see. And that’s what makes the future so exciting!