Most TVs and monitors have a fixed refresh rate, often 60 Hz these days.
If a game runs at, say, 48 FPS, some frames would be doubled every second, and some won't. The resulting video would seem more jittery compared to 15, 30 and 60 FPS, where frames can be doubled or quadrupled evenly.
That's why FreeSync and G-Sync monitors are a big deal to enthusiasts, they avoid this issue for their refresh rates are exactly what's being rendered.
But every modern TV today are not at 100 or above ?
They would be at 120 Hz, but no, they are not. 60 Hz is still the standard for economy models. If you're seeing a really nice TV selling for a huge discount you might want to check that feature to make sure you are getting a good unit. Not that my eyes can tell the difference between 1080p, 4k, 60 Hz or 120Hz, or whatever, but some people really care about these things.