As a relatively fast guy, I definitely enjoy the merits of a nice 5K. One of my favorite distances since I started racing, it is a staple of my racing calendar. Part of my passion for them is just the all out speed entailed. When I am really feeling good, I can consistently run low 16s and that feels amazing. There is nothing like the feeling of finding that rhythm and letting it carry me through the race. A key aspect for me is to avoid splits and pacing markers though. To actually know the times involved can put me off. It is also a flexible distance, good for an all out attack to measure performance, or a good tempo as part of a double race weekend.
The relatively short 3.1 mile distance also means it can be used as a quick fitness test, coming back from an injury or sickness. Long enough to judge capability, but not so long it can put you back at risk, there has been many times I have come back into racing with a tester 5K to see where I stand before going all out. It is also generally the measuring stick for beginners, as evidenced by the many couch to 5K programs. Almost every event on the calendar either features or includes a 5K so it is safe to say that distance will not be phased out anytime soon!