The quality of abrasive materials is difficult to judge by just looking at the product. For instance, your nearest DIY store will probably have a fine selection of abrasive materials. None of these will, in all likelihood, be suitable for valve grinding.
Abrasives on a paper or cloth backing are generally intended for use with traditional belt or disc grinders. In this type of operation, each individual grain is only engaged in the cutting operation for a fraction of a second, before moving away from the working area. It will have plenty of time to cool down before returning to work.
Grinding dust is removed from the area by the same action, keeping the grains clean.
Nearly all the heat generated by friction is removed with the grinding dust, contributing further to the cooling of the abrasive material.
All of this has a positive effect on the service life of the grains, helping them stay sharp for longer.