A lightweight port-wrapper around IP.
For short request short response protocols like DNS, DHCP, NTP, SNMP. where typically each request and response needs only one packet. It's either uncritical if the response doesn't arrive (typical for periodic protocols like SNMP, in which case just wait until the next period happens) or send the request again (a few times, before giving up).
And for various streaming applications where the overhead of TCP would slow down... packets that get lost or out-of-order are just ignored, causing dropouts in the application.