It's a lubricant, much like the molybdenum disulfide that was so popular a few years ago.
What people found with "moly" was that it did reduce bore friction, but that often reduced velocity as well. Powders are designed to for the "standard" friction a bullet would encounter. With less friction, the pressure may not reach maximum before the bullet gets out of the barrel. It seems backwards - like less friction should make the bullets go faster - but reality is somewhat different.