Whats their purpose?

[Spider]

it's when I rain down with snobbery! 

(hey, I thought I spelt it right...no little red line)

condescension vs. condensation 

[Justo]

yep meant the teeny weeny lotsa fine grooves. so thats the consensus. machinery wear, cheaper to keep them rough. so why even bother I wonder, why half smooth and half groove. Smooth and groove! Shake n bake baby!

Sorry got carried away there...