For "movies" it is the most meaningful for when the content is 24Hz, because on the 60Hz displays you will always get frame insertions / repeats, in scenes involving motions it can look stuttering, whereas on a ProMotion display it will play frame by frame as intended.
On Macs the scrolling or interfacing smoothness has been harder to be noticed than say on the iPad for two reasons: on the iPad you use fingers or pen directly touching the screen, you brain expects or even rely on responsiveness under such case. On a cursor based OS where you probably need that fast of an input when swiping or scrolling a PDF with the trackpad, and even then the delay can be dragged.
The second reason is software support. Monterey has been slowly getting or enabling 120Hz mode for various apps or global UI elements. So across macOS & apps you don't see full 120Hz as often as you would on iPadOS and the differences add up.
I somehow feel that ProMotion is more useful when it down clocks to 10Hz, on a battery constrained laptop that does give you minutes if not hours of life depending on what you do.