Inside Camera Zoom allows you to zoom the camera in and out while inside a vehicle using your mouse scroll wheel, just like you would on an outside camera. Looking at that distant corner of the field, or at that small gauge on the dashboard, is now only a scroll away. Clicking the middle mouse button will reset the zoom level (Field of View) back to default.
The 'Reset Vehicle Camera' game setting, if enabled, also resets the view back to default when you switch vehicles.
Modders can use two optional new parameters, 'fovMin' and 'fovMax' on an inside camera to control the upper and lower FOV limits, with a default range of 10-100 degrees. A new console command 'pfToggleInsideVehicleCameraDebug' can help you finetune those values, by displaying the current FOV for the camera.