Unlike applying paint, epoxy overlays need to penetrate and bond mechanically to concrete in order for them to adhere properly.
Often, concrete for a garage is not finished to provide a smooth and even surface. This finishing process reduces the porosity of the concrete. As a result, the thick and viscous nature of coatings cannot penetrate and soak into the surface very well.
Grinding to create a mechanical bond, exposes the pores in concrete so that the epoxy coating can penetrate the surface in order to get the best bond. It also creates a rougher surface for the coating to adhere to. If your concrete is not ground properly, it is often the reason why floor coatings fail and peel, or delaminate.