I've never really speculated into it too much, but i know my hair is really easily prone to buildup and almost every styling product i own has at least one polyquat. although i cant feel the buildup, i'm assuming a clarifying rinse or wash helps remove any unfelt/noticed buildup helping the curls spring up.
also, my hair is prone to overconditioning and overconditioing causes your hair to be fluffy, flyaway and not clump. i think fine curly hair, such as mine, actually needs a little harshness or lack of conditioning to make it clump and curl. without a cleansing wash at least once a week my hair will just be in too good a condition so that it is fluffier and hangs more limp than bouncy tight curls.
thats just my theory but all i know is cleansing once a week does help my curls, so i just do it