Worms are great - no question. Earthworms aerate the soil and composting worms consume decaying material that many consider "garbage." All worms provide excellent nutrition for plants through their castings (or poop).
But what is it in the castings that is so beneficial to plants? They are actually quite low in the "traditional nutrients" of nitrogen, potassium and phosphorus when compared to commercial fertilizers. (Of course, chemical fertilizers include levels that are too high because they're in a form that's difficult for plants to take up, so some of it washes away before it can be absorbed by the roots.)
If key nutrients aren't real standouts in castings, why are worm castings so great for plant growth and vitality? The worms hog the spotlight, but the "secret" is really an army of much smaller organisms doing the heavy lifting.
Microbes are the unsung heroes.
Countless microscopic beings — from protozoa to bacteria to fungi — are present in worm castings and these "good microbes" (as opposed to the bad bacteria such as germs and pathogens) provide essential minerals and nutrients to plant roots, in exchange for root exudates from the plant that are a food source for the microbes.
One other "secret" the worms aren't totally forthcoming about: They aren't the only ones in a worm bin consuming food scraps. Again, bacteria and fungi start the breakdown process, making the food soft enough so worms can eat the slurpee-like delicacies, as they have no teeth. And scientists believe the worms are as interested in the microbes as a source of food as they are food scraps like banana peels.
The secrets are out. Sure, worms are beneficial, but they couldn't do it without the quiet, behind-the-scenes work of microbes.