The angle your screen fills
Your eyes see a wide cone, and a screen fills part of it. How much depends on the screen's width and how far away you sit. A 27-inch 16:9 monitor at 24 inches (60 cm) fills 53 degrees across. Move back to 31 inches (80 cm) and it fills only 41. A 34-inch ultrawide at 28 inches (70 cm) fills 59, and a 49-inch super ultrawide at 31 inches (80 cm) fills 74.
For scale, the visual field of an average person is about 170-180 degrees. Even a super ultrawide leaves most of it empty. Seat references like THX (40 degrees) and SMPTE (30 degrees) use this same angle for TVs.
The angle a game draws
A game has a field of view of its own, set in its options. It's the angle the virtual camera sees, and it has nothing to do with your seat. Games give it as a horizontal or a vertical angle, often without saying which, and the two convert through the aspect ratio. A 90 degree horizontal angle is 59 degrees vertical at 16:9 and 45 at 21:9. So a setting of 90 shows a different picture depending on which angle the game means.
Console games typically use around 60 degrees, and PC games 90 to 100. The reason is distance: a TV sits far away and a monitor sits close.
Why a wide setting shrinks things
A wider camera angle fits more into the frame, and everything in it gets smaller. Going from 90 to 110 degrees shows 43% more width on a flat plane 66 ft (20 m) ahead. A target in the middle of the screen shrinks to 70% of its height.
The other direction has a cost too. A narrow angle feels like looking through binoculars and can cause dizziness or nausea in some people. A wide one can make movement feel faster. There's no single right value. It's a trade between seeing more and seeing targets larger.
Matching the game to your screen
A game looks most natural when its angle is close to the angle your screen fills. A 27-inch monitor at 24 inches (60 cm) fills 53 degrees. A 90 degree game packs a wider scene into that space than your eyes would see from the same spot. Plenty of players accept that on purpose, because the extra view is worth the smaller targets.
The FOV Visualizer draws two angles over the same scene, so you can see what a change in the setting does before you try it.