Grindstone dresser tools are essential
Re-shaping or truing the wheel
If the grinding wheel has gone out of shape due to wear you can use the dressing tool to flatten the grinding surface or trim the wheel back to true running. If you fit a new wheel you will probably have to true the wheel before use.
Refreshing the grinding surface
The little particles of grit on the grinding face of the wheel eventually become blunt and rub rather than cut the tool you are trying to sharpen. This causes harmful heat which can ruin the tool. So occasionally you will have to use a dressing tool to remove the blunt grit to expose fresh new sharp grit. Only one layer of grit has to be removed. The better quality wheels need dressing less often so will last a lot longer. The grey wheels on cheap grinders will need dressing frequently to avoid burning the tool edge.
Note - never dress a CBN or diamond plated wheel as they only have one layer of grit.