For those of you who don't know, my wife cuts hair. She will be annoyed I reduced it to hair cutting, but we essentially do the same thing, just with different tools. In the vernacular of hair, cutting grass with a dull blade will result in a lawn full of split ends.
Look closely and you can see the white torn tops on all the grass blades. It doesn't matter if you do everything else correctly, if you mow the grass with a dull blade, you will have a dull white color to your grass, instead of the deep green it could be.