I believe it's implementing DNS-SD for network printer auto-discovery. I'm not terribly familiar with DNS-SD, but given that normal DNS is UDP based it would be unsurprising for DNS-SD to also use UDP.

DNS is actually UDP/TCP. It’s probably required for receiving unicast messages, if it’s using DNS-SD

The purpose of cups-browsed is to listen on a UDP port which allows it to receive broadcasts from legacy cups servers on the local network, whereupon it will talk to cups and configure a local print queue for the printers on the discovered server.

A modern setup doesn't need it and doesn't use it.

To receive multicast messages, probably.