Why use a Camera instead of Null Object?

I assume you are asking for the rationale or benefit with Parenting a Camera to a Null Object.

This is an intermediate level topic - just so you know. When a Camera is Parented to a 3D Null Object, the Null Object can act as a pivot point for the Camera. This can be achieved by rotating the x or y axes of the Null Object. During this rotation, if the setup is done correctly, with respect to the other layers, then the Camera will be looking at the appropriate layers while it is rotating. To achieve the same effect with just a Camera is going to be a difficult task.

The example above is a simplified, yet powerful, set up for animating cameras. Such a device or setup is referred to as a camera rig. There are more complex camera rigs that are used by those that animate with the camera often.

The second tutorial in the list on this page should give you a better understanding.
Top 5 Adobe After Effects 3D Camera Tip & Tricks — Motion Tutorials

Hi

The difference is that the Null can be placed as a parent not only of the camera but also of other objects so that everyone moves at the same time with the camera, having added to Null tracking parameters for example. Moving the 3D object is not the same as moving the camera or moving several objects with just one Null, you have to experiment with each movement doing your own tests.

I hope I have clarified, even though it is a bit confusing what you ask my friend.

Regards!

It's totally not clear what you actually mean. You need to explain better and use a proper terminology.

Mylenium