If your FPS is 150 on a 144hz monitor, you get stutter. It's all about sync. I really can't wrap my head around explaining it with words, but visually it would take no time to explain, a drawing would be enough. I can try with idiotic low numbers to save space:
TOP - 5 fps (monitor)
------|------|------|------|------| (35 chars)
----|----|----|----|----|----|----| (35 chars)
Bottom - 7 fps (game)
(HTML format make the lines look different length, but they aren't. For better view, copy paste to notepad)
The further the "|" on separate lines are from each others, the more stutter you get. So the higher FPS ingame you have, the closer the "|" will be to each others, removing stutter. The screen picks up the closest already rendered (IE, same row, or to the left) frame "|". You can lock it to your screen, but one single fps dip will push the entire bottom line (game fps) further right. And the sync needs to be dead on for it to work. Most screens aren't 100% perfect deadon to the FPS it shows. (example 144.02 instead of 144)
It's all about trying to create sync without using vsync.
That was the monitor/game. You can repeat same scenario from the combined result, and your eyes. And as for the eyes, no one knows for sure how many FPS is seen.
I know it ain't easy to understand, as it ain't easy to explain.
This is also why I love programs like smooth video project, which converts movies to higher FPS live while they play, making them much more fluid. Once you try this you will dislike watching any movie below minimum 60fps.