I use one on my Phillips/Paterson enlarger.
--Vastly superior film flatness. Really. Your enlargements will be sharper.
--Flexibility. Glass carriers that have four-blade masks let you handle all sorts of formats with the same carrier, and you can use the blades to crop in the carrier, reducing light spill in the darkroom and improving contrast.
--Dust. Four more surfaces to attract dust.
--Newton's Rings. Which are still sometimes a problem with AN glass.