Why is 0 not a valid value for a PerspectiveCamera's near plane

The PerspectiveCamera doc says

0 is not a valid value for a PerspectiveCamera’s near plane.

Why is this?

There’s some discussion here.

Essentially, it would cause this equation, which is used in the setup of the projection matrix, to return 0:

-(2 * zFar * zNear) / (zFar - zNear)