Camera depthbuffer question.

What exactly does unity camera depthbuffer do?

I have a minimap camera and a rendertexture on it.

Currently, my rendertexture's depthbuffer is set to 0 and with such settings, only the map is drawn on the minimap. No player or monster or anything that is not 2D. Even when they are in front of the map.

With depthbuffer set to 8, player and other 3D objects are rendered.

So, what exactly does this depth buffer do in Unity?