4A Games, the developers of Metro: Last Light were receiving a lot of criticism for not having a FoV slider in their game. Community manager Maurice Tan defended their decision on the Steam forums as follows:
The main reason for maintaining a fixed FOV is because we have 3D
elements like the watch and weapon ammo that need to remain visible.
In addition, all the game's first-person cut scenes and cinematics and
each and every animation involving Artyom's hands - idle weapon
animations, reloads, ladder climbing, melee attacks etc, - were
created assuming the same, fixed field of view.
Changing the FOV would break all the cut-scenes and animations - you
would be able to see inside Artyom's arms, or they would appear to
float in the air in front of you. Or worse.
We had considered offering three FOV pre-sets, but this would still
require significant work to re-do every animation, adjust the HUD and
UI and other seemingly small but incredibly time consuming tasks.
Even with a wider but still fixed field of view, Artyom's hands would
look too far away. We know - we tried.
Game performance is also tied to FOV - the amount of geometric detail
we put in each scene has been partly determined by this set FOV, and
setting a wider FOV would have a performance impact.
The main reason for not having a FoV slider was that they were going for a no-hud high-immersion approach were all information important for the player was displayed on 3d elements in their field of view. When your GUI is part of your render-scene, changing the FOV changes your GUI.
Something Tan didn't mention in above post but which likely also was a part of their decision is that the FOV is important for level-planning. Metro: Last Light is a horror-oriented game with a lot of jump-scares and other shock-moments. There are lots of situations where the player is distracted by something interesting so they look into one direction, and are then surprised by a sudden attack from the side. This shock-effect can only work when the level designers know the FOV of the player and so know what they can and can't see when looking from a certain point to a certain other point.
The same problem also appears in a different way when directing cutscenes. When you have high-immersion first person cutscenes, you likely want to have full control over the camera to be able to control what the player can and can't see. But when you don't know the FOV, directing the cinematography properly can be impossible. When the FOV of the player is higher than you expected, they will see objects they are not supposed to see yet. When the FOV is lower, the player won't see things they are supposed to see during a cutscene.