This is useful for taking screenshots of your truck, especially when you have FXAA injectors active as the photo editor will not do so well with that.
But it's also useful when you need to get your truck out of a bind.
Say there's an AI glitch where the AI gets stuck at an intersection and refuses to move.
Entering developer camera mode you can look at where the traffic is stuck and you can also move the camera past that location, close to the road surface and pointing in the direction you want to be traveling.