Do not ignore the graph
Well firstly it is of vital importance for your situational awareness.
Secondly you might have dispatched the aircraft by MEL with a start valve inop.
Then if you suffered an engine failure, and attempted a relight in the start assisted envelope , you will not understand why the relight was not successful ( start valve inop)
Conversely , if you knew your envelope you could anticipate and plan forward for a speed that falls into the windmill envelope in order to have a successful relight at first hand , and avoid an unduly unsuccesful start attempt.