The tail is fully prehensile and strong enough to support the whole body, and it carries dense touch receptors under the bare patch, giving the animal fine feedback about what it is holding. The ridged skin works the same way as ridged skin on primate fingers, improving grip on wet or smooth bark and helping detect slippage.
The hand went the other way. Spider monkeys have no functional thumb, or only a vestigial nub, so the four remaining fingers form a simple hook for swinging beneath branches. Grip precision is handled by the tail rather than the hand.
Prehensile tails of this kind are found only in New World monkeys, and only in a few groups within them: spider, woolly, howler and capuchin monkeys, with the bare friction pad confined to the first three. No Old World monkey or ape has one, and the trait evolved after the two lineages separated.