Problem:
Track is playing and you noticed an interesting point that you want to set a cue point at. You cant go back because the track is still playing and you forget to do it afterwards.
Solution:
You enter the Beat Grid Edit mode, stop the movement of the waveform (while the track is still playing), and move the waveform back to the position where you want to enter the cue point, you enter the cue point, and return to the track.
PS: The idea to be able to stop the waveform in the Beat Grid Edit mode and move it around, while the track continues playing, would be handy anyway to make on-the-fly changes to the beat grid (for example when the grid is slightly off-beat)
I would suggest a PAUZE or FREEZE button in the Grid Edit screen. Making an instant double has its limitations (when you are playing tracks on both layers)