They’re way more comfortable than you think
Unless you're wearing UFO pants, stuffing your pockets with a phone, wallet, keys, Tic-Tacs, and lucky rabbit's foot can make it tough to both sit and walk; the fanny pack eliminates this issue entirely.
As an added bonus, its central location on your body alleviates the back or shoulder strain that can come from toting a more traditional murse or messenger bag. Also, if you've carrying a bigger messenger bag, you're gonna be tempted to fill it with more messages. Or, other stuff.