Because with vsync off your client is trying to render as fast as possible even if there is nothing new to render which pegs your CPU and/or GPU at 100%, impacting performance of other things, such as your VM, and starving threads for cursor updates, etc. A11 includes a FPS limiter to combat this for those that insist on running with vsync off.
Please note that VSync doesn’t exist just to reduce screen tearing, it’s main feature is to prevent your system going nuts trying to do extra work for no reason. If you must run without vsync, anything greater then 2x the native refresh rate of your screen is useless, wastes power, generates excess heat, and impacts performance in other areas.
In short, VSync is a good thing and before people turn it off they should learn what it is for, what it does, and why. Once you are rendering at the refresh rate of your monitor, anything more is just useless.