Rationale :
There is literally no difference from 70 fps to 900 fps on a 60hz monitor, because a 60hz monitor is only capable of displaying up to 60 fps.
On a 60hz monitor, the difference from 30 to 45 fps is exactly the same as it is on a 120hz monitor, or 144hz monitor.
The refresh rate does absolutely nothing to the framerate, other than limit it.
That's not quite true on either account.
On a 60hz monitor, when your FPS go well beyond the refresh rate, you get more up to day partial images, which also result in tearing. The last part of the screen to be updated is newer than what you'd have with lower FPS, and when the next refresh starts, the top part of the screen is that more recent image as well. Of course tearing can be annoying to see as well, but there is a difference.
And having a higher refresh rate, with low FPS, helps in 2 ways:
1) With V-sync, you have more frame time options, meaning that V-sync will not have to make you wait as long to show the image when it is force to wait for a new refresh before it shows it.
2) Without V-sync, the refreshes remove tears sooner than at a lower refresh rate. The tear is there when a new frame is updated, but not on refreshes that happen between frame updates.