"No mow May" to save the bees/over-wintering insects - is now almost impossible as you'll have more than a foot high of regular lawn grass before June 1.
I'm guessing that the lawn itself is incidental, and by not mowing you're letting flowers (including dandelions and clover?) grow?
Also, I'm gonna be up front about my ulterior motive: I hardly mow my lawn anyways and would love to have another reason to justify / rationalize what I'm already doing :)
But still - genuine question.
There's debate about how effective it is, but that's the theory at least.
The perfect lawn is a relatively new thing - blame Scott lawn and other lawn-care companies.