I'm the garbage blog dude, thanks Meghan for sharing it!
Empty boxes are indeed worth money. Not sure why exactly, but it probably has something to do with resale value, plus they are really sturdy and good to ship in.
Some vintage boxes are worth really good money. A while back I found an old Tudor (basically a lower priced Rolex, made by Rolex) watch box that was in really nice condition. It sold for 180$ if you can believe that. I've also sold vintage Omega watch boxes for between 50 and 80$, and that's despite them having dirty / no interiors. An old Tiffany box went for 50$, newer Tiffany boxes go for around 20$, the list goes on...
It's really hard to believe what people pay money for. I've learned most of what I know by looking up everything (well, not everything but a lot) on eBay's completed / sold listings. Almost all the info you ever need will be there, and if you do it often enough you can start making money regularly from flipping or garbage picking.