I know lol
For the last time, it's benefit is to remove screen tear and input lag, but has the same trade off has vsync that it can stutter real bad if you drop the frame rate below refresh rate.
It doesn't matter if it don't limit the frame rate. It's method is very similar.
FastSync
144hz @180fps = perfectly matched frames @144fps = no screen tear or input lag. Chance of stuttering
Vsync
144hz @180fps = perfectly matched frames no screen tear but does have input lag and stutter.
Limited frame rate. With either Gsync or freesync
144hz @180fps = perfectly matched frames no screen tear, no input lag or stutter.
End of the day, it doesn't matter if it's limited frame rate the outcome remains the same. It's just another method of removing vsync stutter & input lag only any use to someone without Gsync or freesync.
You are missing how Fast Sync works - in a traditional vsync and/or vsync with frame limit situation the game simply doesn't produce extra frames and can take additional time to recover from dips in framerate - with Fast Sync the game keeps ticking over as fast as it can while not all those frames are used some are and the result is lower input latency and faster recovery from low framerate situations.