motogp brake rotor covers - why and when?

 i've seen some pics of bikes from the Assen motogp event.

i saw a couple, krjr and capirossi come to mind, where they were
running some sort of cover over the front rotors.

what's the purpose of these covers?

protection from debris?  temperature mangement?

given that it wasnt on every bike, i'm guessing it's the latter.

do some brake setups need the rotors to (ironically?) not shed too many
BTUs to maintain temp and thus brake effectiveness?

 wamanning@gmail.com wrote:

I believe Carbon Fibre brakes need the covers to keep the brakes up to
operating temperature.

 

Doubtful they were carbon fiber, prolly carbon/carbon, think leading edge of
the space shuttle, or reentry vehicle and yes, they not only don't work for
shit until heated, but don't retain heat very well either. It was cold that
day at PI, hence the covers...