As you use your bench grinder, the grinding wheel wears away. If you don’t adjust the spark guards and eye shields, you could find yourself showered in tiny red-hot particles! Adjusting the tool rests helps maintain optimum performance. If the gap between the wheel and tool rest gets to big, you get excess vibration, and the wheel won’t cut as well as it should. There’s also the chance of the workpiece snagging on the wheel and getting pulled downward, which is very dangerous. The manufacturer’s instructions will tell you what the gap ought to be. Adjustments should be made regularly.