Your frame rate is determined by the capability of your cpu to generate frames, the ability of the gpu to output frames and the ability of your monitor to display frames.
G.sync and such try to address the mismatch among all of these capabilities.
And, there is another factor, input latency that may be impacted.
Here is a nice article that explains the interactions:
https://www.anandtech.com/show/10325/the-nvidia-geforce-gtx-1080-and-1070-founders-edition-review/13
g.sync applies to nvidia graphics cards. AMD has freesync.
What is your cpu, gpu and monitor?
What kinds of games are you playing?
There is likely a way to match capabilities with the equipment you already have.
As to the worth of a 144hz monitor, that is something only YOU can determine.