I do most of my commuting and joy riding at 15-20 mph. Occasionally, for momentary reasons, I have gone faster, as high as 31 mph. Those moments are rare, and a lot of fun. Back when I was in better shape and commuting on a pedal bike to work, I would keep my speed between 12 and 15 mph so as not to be so sweaty when I got to the office. So being able to run 15-20 mph, also without working up a lather, is an amazing benefit AFAIAC.
Keeping it in the 15-20 range also helps to maximize battery charge, and minimizes wear and tear on the bike and all of its components. Also, at speeds over around 20, more of the motor's energy goes into overcoming wind resistance than into increasing speed, so it takes more power to produce less result.