A sharp blade helps keep the lawn healthy. When the blade begins to dull, you’ll notice that the lawn looks uneven. But it’s more than just aesthetics, mowing with a dull blade will shred and tear the grass affecting the grass’ ability to absorb water and nutrients, and potentially opening the lawn to infection. A dull blade can even rip the grass out by the roots which will contribute to weed growth.
A dull blade affects the mower's performance. As well, there’s additional stress on belts, bearings, and the spindle housing (which holds the blades on). A dull blade also uses more energy than if the blade were sharp. Gas mowers will use more fuel, and if you’re using a cordless electric model, the extra strain will have a significant impact upon the runtime you’re able to get out of each charge.
Get the blade professionally sharpened at least once a year. Professionals have high-precision grinders and the technology to set the blade precisely against the mower's cylinder.