I would assume that parkour is helpful if one happens to be in a situation where they need to quickly escape from vicious dogs and the only way out is jumping over a super tall wall. For regular folks it definitely strengthens your core, arms, legs and butt.
Now that people are learning new things faster, and recording what they are doing, it challenges others to do something more advanced, in other words, "one-upping" and that ultimately continues the trend.
I've been waiting with something akin to bated breath for the second season. Bravo can do reality competition programs like no other network, it's way clear. They have the formula down and they're really good at it.