If you set ffxiv to use gsync you get

a lot of problems with sounds
physics problems (camera shakes)
battle effects (the biggest problem of all): when gsync is on you lose a lot of fps, specially with higher resolution (gsync turned off and vertical sync on fixes this problem)

Btw these examples are with vertical sync off (and only gsync) as well as with both on.

I know people will ask for computer specs but instead of that why don't you try turning off gsync and see how your fps/response and fight physics feel when you only have vsync on. You will notice it, specially in big fights like in raids.