UDP is used when low latency and low jitter is more important than complete information.
For telefony it's better to lose a few packets than to wait a second or two for a retransmit. Some for game state, I believe.
And I do believe that TCP was also designed by network engineers :-)
Rather, you'd want to use a protocol built on UDP whenever you want something faster, better, more reliable than TCP.
UDP is used when low latency and low jitter is more important than complete information.
For telefony it's better to lose a few packets than to wait a second or two for a retransmit. Some for game state, I believe.
And I do believe that TCP was also designed by network engineers :-)