"Worm Castings aka Earth Worm Castings aka EWC (we'll call them EWC for ease sake) have nutrients that are far more readily AVAILABLE to your plants, the USA Department of Agriculture states that on average EWC contain approx. 5 times the exchangeable Magnesium (Mg), 7 times the available Phosphorus (P), 11 times available Potash/Potassium (K), and 1 and a half times the Calcium (Ca) found in fine top soil.
And these nutrients are in a state that can almost instantly be used by your plants, as they need it.
The N-P-K and trace elements vary hugely depending on what the worms are fed. Store bought EWC, u don't know what NPK ratios your getting, but if you make your own, you can get feed them certain foods to achieve the desired NPK ratios, a long process, but a useful one if your in to saving the planet and all that haha.
There are websites on Worm Farming/Cultivation, Google them and some tell you what foods to feed your worms to achieve said desired NPK ratio, also you can usually find a local-ish wormary where you can order worms and worm bins/farms from and have them couriered to you. (Well maintained Worm Farms don't smell so you can even keep them in the garage if you want)."