In many situations, you only need one camera in Unity or similar game-development tools. The 3D models share the same camera as the canvas where parts of the UI might be generated. In Unity, the canvas is always on top of the 3D models when in default “Overlay mode”. However, you can change the canvas to “Camera mode”, which allows you to put 3D models both in front of and behind the canvas.
But when do you need two or more cameras?
One good example is when you want to mix perspective and orthographic views. In our next game, we want to draw on the left side a 3D world in perspective. On the right side, we have placed a standard canvas. The canvas is in “Camera mode” so we can add 3D models in front of it, in our case, a cute rabbit.
If we used a perspective camera, we would see the rabbit from below when it is at the top of the display, and from above when it is at the bottom. In many situations, this is what you expect and want, but as the rabbit is interacting with the numbers on the canvas, which isn’t drawn in perspective, it would look strange. It is much better to keep even the rabbit in an orthographic view. Thus we need one camera for the perspective world and one camera for the canvas and its orthographic rabbits.