Most monitors/laptop screens have refresh rates of 60 Hz. Some can run a bit higher (75, 144, etc.). Let's use 60 as a baseline, but you can see yours by right-clicking on the desktop and Display Settings > Advanced Display Settings.
That means fps rates of higher than 60 cannot be interpreted by your monitor and are the equivalent of digital noise. If a game or other program's actual fps rate spikes up to 65 or 70, that can lead to occasional graphics glitches and some small amount of lag/stutter, but usually it wouldn't really be that bad. With a powerful enough GPU like yours, the frame rates generated can easily go into the hundreds and fluctuate wildly all over the place. With a yet more powerful card, they can go even higher like into the thousands. Vertical sync is supposed to fix that by bringing the game's frame generation "into sync" with the monitor. It doesn't work in windowed mode because without full screen Windows is managing that aspect rather than the graphics card control panel. And we still think it's not working in full screen in your case because of the dual graphics and handoff from the Intel chip to the Nvidia card needing some extra help by way of that Intel HD setting that is eluding you.
Anyway, all that extra frame generation per second provides no benefit and overworks the card hard. Some will experience high pitched whining noises coming from the area of the card within their computers when this happens, which is a huge warning sign that something is wrong. Graphics glitches, lag, and crashes often arise as well or instead, with the danger of the card overheating not far behind, leading the player to think there is something wrong with their game or its added content when there really isn't. Most modern games include a mechanism to sync or cap the frame rates themselves, even TS4 does, but TS3 is kind of an oddity because of when it was designed, before this became more of a standard thing, and how hard the game engine can work in its attempts to render and simulate everything it needs to.
If the card were to overheat to the point where it melts down or explodes, it can damage the system board on which it is attached as well and that becomes either a very expensive repair job or renders the computer useless if the repair would cost more than just replacing it with a new computer would be. Most laptops in particular have an extremely high temperature sensor built in so that besides or in addition to high pitched whining noises and the fans constantly kicking into super high gear, they may find the game shuts their laptop off abruptly if its defense mechanism is triggered. For some reason players don't like suddenly and unexpectedly staring at a black screened, powered off laptop when seconds ago they were trying to play a game or do other things. That impact is of course better than a meltdown or explosion, but it doesn't make the game any more playable until the underlying issues are addressed.
