Minimum take off roll.
Keeping the nose wheel on the ground reduces induced drag so the aircraft accelerates faster and reaches take off speed sooner with less runway, tyre wear, engine life, etc.
Rotating at about 15 knots before calculated take off speed enables the pilot to pull off the aircraft smoothly without snatching and risk scraping the tail.
At least that is what I was told at my Valiant conversion course in 1962.