Depends on where you live; they're everywhere here in Japan (where they were invented).

This is probably due to the evolution of Japanese phones, which have had a crude mobile internet and built-in cameras from about 2000 onwards. Given the limitations of the devices and networks, QR codes were the easiest way to move data between phones (because there was no way in hell NTT would allow you to run apps on one).

These then got used for coupons, and with the shrinking economy and people wanting to save money, people quickly internalized how QR codes worked. Likewise, phones come with QR capability as a builtin -- it's not a separate app.

I imagine you might see the same sort of use case in developing nations where computers and smartphones are too expensive, but that window is rapidly shrinking due to the low cost of Android devices.

Only if you think of them as just encoded URLs; they do solve a myriad of real problems. "Just type the URL" doesn't hold when you want to add query parameters to it, use a different protocol or store data by itself. It's a barcode with more density, so it serves the same purpose as 2D barcodes; it is widely deployed in industry inventory control systems, shipping, access control, event ticketing, shopping.