Originally Posted by
dscheidt
The purpose of sanding isn't just cleaning. It also greatly increases the surface area that's available to bond with the patch, which increases the bond strength.
Sanding also removes any mold release compound which can interfere with a good bond being achieved. The patch is not merely glued on, there is a chemical reaction between the vulcanizing fluid and the rubber of both the patch and tube, which when properly done makes them essentially a single piece of rubber. Regular rubber cement or contact cement does not cause this reaction, which is why they are unsuitable for this application.