Better adhesion to bare metal than any other primer. Better moisture resistance - it's literally waterproof. Years ago I sandblasted and primed two coats of epoxy on a hood cowling on a Honda Accord. I never got around to topcoating it before I got rid of the car. But it was always outside in the rain and snow for several years with only the primer. There was no indication of any rust forming under the primer or any failure whatsoever, except fading of the gray-green primer color.
Does it make a difference since it's under a topcoat? It does from an adhesion standpoint. I don't know about the moisture issue. I suppose it depends on the topcoat and the relative moisture permeabilities. Certainly if a scratch was only down to the primer, the epoxy film would provide better moisture protection to the bare metal than any other primer in the same situation. It's the only bare metal primer I've used for the last 40 years. That's how good I think it is. Others will have their preference but for me epoxy is the only way to go.