What could be simpler than moving a lawnmower over the grass? Why would anyone need advice on that? Well, like anything, there's always more to it than appears at first glance.
Selecting the proper length is always the first puzzle to solve in lawn care. The answer is complicated by the variability in types of grass, average amount of sunshine and rain and other factors. But, in general, the following holds true...
Keeping most grass a little longer saves effort cutting. Allowing the lawn to grow an extra week between cuttings, mowing every two or three weeks instead of every week, reduces the effort by up to a third. That saved hour or two can be well used on other activities - fertilizing, weeding and other needed tasks.
Longer grass will also grow slower, since the length increases at a higher rate at first, then decreases as the blade extends upwards of a couple of inches or more. The longer the blade, the slower the growth.
Keeping the grass longer helps the grass, too, in many cases. Longer grass can retain moisture better, making for lower water bills and better growing rates. Longer blades have more surface area for photosynthesis, the biochemical process that turns sunlight and compounds into energy used for growth and reproduction.
Keeping the grass longer reduces weeds and time spent on weed maintenance. Weeds have to germinate in order to grow, just like nearly every plant. Longer grass blades rob weed seeds of the sunlight and warmth that stimulate that process. That kind of theft is to be encouraged.
Longer grass encourages deeper roots. This is one other way that grass out-competes weeds, since the amount of nutrient and space under the surface is limited. When it's occupied more by grass roots, there's less left over for weeds. Longer roots also help grass reach moisture further down, making the plant stronger and better fed.
There are limits, however. Apart from the appearance of a shaggy lawn, grass that's too long can encourage the growth of lawn bugs and mosquitoes. The latter prefer cool weather, and the longer grass gives them a place to 'hide in the shade'. Keeping the grass down to a moderate length will help keep the bugs at bay.
Also, when grass gets to more than a few inches, it buds at the end, throwing off new seed. This completes a natural lifecycle and the blade then dies off. Unlike some organisms, slicing them actually helps keep them healthy.
Cutting grass doesn't harm it in any way, provided it's not cut too short. Grass grows from the base (or 'crown'), not from the tip. So, provided the crown isn't damaged by nicking it with a blade, it will do fine.
Though the crown is far down and most lawnmower blades are set to between 1-3 inches, if the yard is bumpy it's possible for the blade to dip. At a large enough angle, the tip of a wide blade can dig into a crown.
Keep the grass a little longer in hot weather, a little shorter in cool weather and you'll achieve the perfect balance.