A stump left after tree removal can obstruct mowing, planting, fencing and landscaping. Grinding uses a rotating cutting wheel to reduce the main stump and accessible buttress roots into small wood fragments. It is normally less disruptive than excavating the whole stump with heavy machinery.
“Below ground” is not a complete specification. Turf may need a different depth and backfill from a new path, patio or replacement tree. Tell the contractor how the area will be used so the grinding area, depth, removal of arisings and any imported topsoil can be shown clearly in the quote.
Grinding does not remove the entire underground root system. Wider roots are normally left to decay naturally unless a separate excavation is required for engineering or construction reasons.