There's many reasons for that, such as:
The field of vision in ArmA 3 does not feel natural at some resolutions. You may feel crampled with it.
The default field of vision decreases frames per second. (Holding - to zoom out gives me 2~3 more FPS (Every single FPS counts!))
Brings the player closer to a real human field of vision, while zooming in would still preserve detail for longer distances.
Improved CQC experience.