My understanding is that Mycelium mostly helps with nutrient motility, moving them across roots of the plants. Potentially increasing bioavailability.

Some Eukaryotes are also reported to my mycellial connection with the plant (i.e. merge with the plants) some sort of forest underweb sort of thing, but I think that would be very difficult to reproduce insitu. Eukaryotes are amazing but also so diverse they defy classification.