Why does a runner choose to pin his race bib on his waist and let it dangle?

I can only imagine it is to facilitate apparel laying. It would make it easier to discard items of clothing no longer needed.

Just set your bib free and let it dangle - I ran my best times when I let it dangle.

It's a neccesity for male runners who do races shirtless. It's functional and sends the message to other gay men on the race course they are available.

For women, they hate putting it on their sports bra.

So I can hide my identity when I cut the course short.

To cover my largess.

It's better than pinning on your back