A peer to peer network is built on top of unicasting and takes advantage of the fact that you have relatively low-bandwidth uplink nodes distributed among many peers that then "feed" a high-speed consuming node. The receiver then assembles the chopped up pieces into a larger file which is compared to a checksum to make sure that all pieces are genuine.
Similarely multicasting does the same but allows the receiver to "opt-in". Multicasting is designed to be routed and was historically not popular because of the CPU it took up for no charge of the sender. There is a certain turn-around point though when it is costlier to harbour unicast traffic than multicast.