Why are some ultrahard grits metal coated?

There are two classifications of metal coating:

Metal cladding which comprises a thick layer of typically nickel or copper which can be up to 60% of the total particle weight. These clad particles are generally used in resin bonded tools such as grinding wheels. The effect of the cladding is to improve the retention of the abrasive in the bond and act as a heat sink to help dissipate the heat generated during grinding and hence improve performance of the tool.

Metal coating comprises a thin layer of metal,such as titanium. When used in bonds which use high temperatures during their manufacture such as some metal and vitrified bond systems these coatings can improve particle retention and protects the diamond during manufacturing of the tool/segment.