During a race the bike gets slammed around so much that the fluid splashes around violently in the reservoir. Being brake fluid it can seep slightly, and the thing is to absorb any slight seepage so that the wind doesn't spatter a slime of it onto the visor of the racer.

I've seen a track bike a tiny bit wet around the reservoir (not an NSR), but not enough to need something to soak it up.
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MC28 SE -sold-
MC21 with RS250 engine -for sale-
MC21 race bike -soon for sale-