I asked my husband, the home sport-guru and he told me to ‘ask the dopers’. 
If it’s actually grease paint (as it was originally), it decreases glare from the sun or stadium lighting. If it’s a sticker (as is becoming more and more common), they think it decreases glare.
Old school guys use burnt cork…
It does reduce glare especially if you have high cheekbones
According to Tony Kornheiser it’s called “lampblack”, I assume from the soot that collected on the globes of burning lamps. I would think this would have a lower albedo than grease.
Really hoping I used the word albedo correctly…