Question: what are the benefits of using udp instead of tcp as a transport protocol for dns?
Answer: 1) UDP is much faster. TCP is slow as it requires 3-way handshake. ... 
2) DNS requests are generally very small and fit well within UDP segments.
2) UDP is not reliable, but reliability can added on application layer. An application can use UDP and can be reliable by using a timeout and resend at the application layer.