There’s not much significance to it other than that each “control point” has 4 draggable points. The inner points allow you to drag the envelop so as to increase the gain (make the waveform bigger vertically).
The usual way to remove control points is to just drag them off the track (one by one).
A little “unofficial” trick to remove ALL envelope points from a track is to select the entire track, then run this code in the Nyquist Prompt
;version 4
;codetype lisp
*track*
Note that this relies on the fact that Nyquist does not currently support envelope points, which may not always be the case.