UDP (RFC 768 1980; Comer 1988) operates on top of IP. It is used to send/receive datagrams. Like IP, UDP does not guarantee reliability but it is fast and efficient. It is used in situations where reliability is not essential. For example, a user may use the ping command to probe a target host, as in
ping hostname OR ping IPaddress
Ping is an application program which sends a UDP packet with a timestamp to a target host. Upon receiving a pinging packet, the target host echoes the UDP packet with a timestamp back to the sender, allowing the sender to compute and display the round-trip time. If the target host does not exist or is down, the pinging UDP packets would be dropped by routers when its TTL reduces to 0. In that case, the user would notice the absence of any response from the target host. The user may either try to ping it again or conclude that the target host is down. In this case, using UDP is preferred since reliability is not essential.
Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.