Running backpacks everywhere

As I have watched the number of runners in NYC double/triple/quadruple/etc over the last 5 years I feel like I have also watched the number of runners wearing backpacks double/triple/quadruple over the course of just this summer. In the densest metropolis in America where anything you could possibly need is a cell phone tap or a watch tap or a simple cash payment away, I must know: what are you carrying in those backpacks? And are you not miserably sweaty and horribly chafed?

EDIT: I am talking about both backpacks & water vests.