Was made to wear them as a kid in the 90s, mainly while traveling, but they also were handy to keep a small selection of toys like a Magic deck, your Gameboy etc. in so when going out you just grab it. You could still put it in your backpack when carrying one.
On travels they were supposed to be safer than a backpack against pickpockets but it kind of always struck me as a "hey, I'm a tourist, pick my pockets!" indicator. Maybe wear one with only worthless stuff in it and keep the important stuff somewhere else?
I think they can certainly be very useful but so are military/outdoor backpacks/ammo packs, cargo pants, load carry vests... I don't see it particularly useful when compared to those and their looks would rather put me off than interest me in them. But I guess they share those properties with those hand bags. I still don't understand why so many women carry them around anywhere when their purpose is better served by just using a small backpack... or even a fanny pack.
They probably don't need to look as stupid as some of the examples posted in this thread and I think they might be more popular if they didn't default to stupid 90s (or hand bag inspired) designs for so long. For example they serve their purpose on delivery men and don't look out of place because they have colors and designs that fit with the uniform rather than being black leather with glitter nor purple with green highlights.