The height-velocity diagram is provided by the aircraft manufacturer and indicates speeds that should be avoided as a safe autorotation might not be possible.
Should there be an engine-failure or other issue that requires an autorotation, operating within the shaded areas of the height-velocity diagram reduces the chances of a successful autorotation. As the aircraft changes from powered flight to autorotative flight, there may not be enough airflow to provide adequate rotor RPM to arrest the descent if the aircraft is not high-enough or does not have enough forward speed.
Conversely, if the aircraft has excessive speed, but not much height, the pilot may not have enough reaction time to slow the aircraft to a safe landing speed. Or if the pilot attempts to do so, the flare may be so excessive that there is not enough clearance for the tailboom and it may strike the ground.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-8
Principles of Helicopter Flight, 2nd Edition, pg. 153
Principles of Helicopter Flight, 2nd Edition, pg. 28
FM 3-04.203-2007 Fundamentals of Flight pg. 1-28