To keep your grass healthy and growing properly you should cut the grass to an ideal height of around 2 and 1/2 inches long.
Cutting your grass too short can cause damage to the grass and cause weed invasion so when your grass is too short your grass and lawn is more susceptible to weeds than if you cut your grass at a proper height.
I like to cut my grass when it grows to a height of 3 2/3 inches high.
When cutting grass to keep it healthy and thriving and have a nice lawn you should be removing at least 1/3 of the grass blades.
Cutting too short is bad and cutting too high is also bad for healthy and thriving lawns.