UDP adds only ports and a checksum to IP. There is no connection to set up, so a device can wake, send one datagram and sleep, and the sender keeps no state that could go stale while it is offline. Packets may arrive out of order, duplicated or not at all, and any application that cares must handle that itself.
Those properties fit battery devices on lossy radios, which is why CoAP, LwM2M, MQTT-SN and DNS use it, and why 6LoWPAN compresses UDP headers specifically. DTLS provides security equivalent to TLS over datagrams. The trade-offs are that many corporate firewalls and NATs treat UDP flows with suspicion or time them out quickly, and that reliability has to be rebuilt in the application, as CoAP does with confirmable messages. For large payloads or long sessions, TCP remains simpler. See CoAP explained.
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