Ahh in rhythm games on fast + more advanced stages, there are waaaay too many notes close together.
In Gitadora drums, sometimes you have to hit 16th note or 32th note drum rolls/fills at 140bpm, which would look like a solid line.
Speed multiplier would scale/spread the notes out. They would approach more quickly, but it'll be easier to distinguish the order of the notes due to the increased space between them.
Here's an example
He's playing a 4x speed, causing the notes to be 4x apart. If it was 1x, the song would be barely readable due to everything being so clumped together.
I forget if beatsaber has a speed multiplier option... But since you have the bonus of depth perception, and that sometimes blocks are occluded by others it may not be too useful in that game.