It also can help improve graphical quality and performance overall.Why a cap at all? The only thing you will protect are your ears.
Modern GPUs will throttle from heat and it takes some time to transfer that to a heat spreader and then to a cooling system. The load in a game tends not to be even so it is constantly making adjustments and cooling lags behind these adjustments. You could run with 100% cooling all the time but you will have wind tunnel level noise that will distract from your gaming experience as well. Even a water cooling system will have some fans that are going to be loud at 100% speed.
It is also wasteful of energy which will not only increase your electricity bill but can also shorten the life of some components such as fans that have a shorter MTBF when run at max speed. The extra waste heat will also require more elaborate cooling or otherwise build up in your case which could cause throttling.
Lastly the cap also smoothes out performance at extremely high fps which can make it more visually appealing. It can help avoid rapidly changing fps and related poor frame times.
tldr: A cap can make your system work better overall.