Those disposable cartridges have two points of contact with your face, the blades and the back part of the cartridge. So the blade is lubed by your shaving cream, but surprise! The cream is shaved off! So the strip is to keep the back of the cartridge slick.
Old school double sided or single sided safety razors (not the knife ones!) only touch your face with the blade and the small strip of metal right behind it.
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