Kangaroos have it right when it comes to carrying valuables; they carry their young in a front pocket. A human equivalent of this unique practice exhibited by these furry creatures is the butt pack. Our version has a couple of distinctions from the animal from "down under," though. It is positioned along the waist either in front on the stomach, or in back above one's butt. Additionally, its contents are usually valuable financial items along with everyday necessities like cell phone, keys, etc. This human pouch has advantages over using a purse or wallet for a portable form of storage.
More Health Conscious
Health is the number one reason that any of us should choose a different way of doing things. For men, especially, the butt pack is a step in this direction when it comes to carrying cell phones. Cell phones carried in the front pocket of jeans, pants, or shorts exposes your "privates" to a level of radiation that is undesirable and possibly damaging. More specifically speaking, sperm production is adversely affected by this common practice as expressed by health professionals. The cell phone is safer in a fanny pack as it places this radiation at a farther distance from this sensitive area.
Thieves are less apt to target a woman wearing a pack placed on the front of their waste compared to a purse draped over their shoulders. Men whose usual habit is to place their wallet in their back pocket would also benefit by wearing a belt bag. Normally, they would be prime pickings for pickpockets who seem to clumsily bump into you in a large crowd while walking away with your possessions.
More Compact and Space Efficient
It is much easier and more accessible to have one of these on your person if you are going through security at an airport. The clip to the belt part of it can be instantly released if any questions arise to its contents. Airport security can scan it without the usual task of emptying out every useable crack or cavity in your outfit.
Lastly, it is invaluable when you are participating in a sport or activity where pockets are non-existent or inaccessible. Bicyclists, for example, wear polyester shorts that have no pockets. This is an ideal solution to that problem.