The role of sweat appears to be to make the keratin in the finger softer and allowing more of it to contact the hard surface. This is unnecessary when contactin

The role of sweat appears to be to make the keratin in the finger softer and allowing more of it to contact the hard surface. This is unnecessary when contacting a rubber surface since the rubber surface is already soft. I.e. rubber makes the sweating unnecessary and hence works even against hard surfaces like non-sweaty keratin or asphalt.