Essentially the biggest reason they are used so much is because of the argument "why not?"
I was in a meeting with the organisers of a pretty big consumer tech event, a couple of clever, successful people, and they mentioned that they were going to be putting QR codes on nearly all branding at the event we were discussing. I, slightly rudely, chuckled and asked why on earth they would do that; the answer: "We've found a site where you can make them really, really cheap - like, almost free - so we might as well!"
I was in a meeting with the organisers of a pretty big consumer tech event, a couple of clever, successful people, and they mentioned that they were going to be putting QR codes on nearly all branding at the event we were discussing. I, slightly rudely, chuckled and asked why on earth they would do that; the answer: "We've found a site where you can make them really, really cheap - like, almost free - so we might as well!"