if u live where it's dusty, the sweatbands would deter the dust from clinging to the reservoir cover, hence to protect your fluid whenever u open up the reservoir.
sometimes, when u open up the reservoir cap for inspection, dust that is already clinging to the rim of the cap will get loose, and wallahhh...falls into the fluid.
$5-$20 for sweatband OR flushing your brake/clutch fluid frequently...
i use old socks, it covers everythimg...