A zero fret acts as a string nut would-except that instead of being made of bone, brass, or graphite, it is made of fret wire. Reason: so open strings sound the same or much like fretted notes. There is usually a nut behind the zero fret that acts as the string guide to align the strings for proper spacing over the zero fret.
My personal view is that a metal nut or zero fret is about equal in affect.
By the way, you may note that the zero fret is a hair taller than the first fret to allow proper clearance of the string over the first fret.
Bryant