[sweettea]

A high quality grinder also minimally heats the beans as they grind - heat can change the volatility of the oils, resulting in changed flavor (and usually not for the better)

[FrankoDelMar]

Correct, this is why people prefer burr mill grinders made of ceramic instead of metal since ceramic is more thermally insulating.

[roflyear]

This isn't true in the high-end coffee community. No one uses ceramic burrs, and they are generally considered to produce worse coffee than other burrs.