Yeah, the use cases are pretty slim. Link to your app on a flyer or in a location I'm going to be interested in it is the one use case for QR codes I might have used multiple times, but in general that also is a sign of the inability to create a nice short-link instead.

Sometimes the apps are useful for barcodes that are already there: I sometimes scanned ISBN codes on books that look interesting in the library or in stores, and used book sellers have apps where you simply scan the books you want to sell and get the price.

All in all, QR codes scanned with a phone are not that much nicer than typing something to be that valuable, even if you've found something I actually want to interact with. Start the app, find a position where the code is in the frame and the camera can focus, wait to focus, wait to decode the code and display the result, do something with it... takes quite a while as well. Normal barcode applications work so well because the scanning hardware/process is fast.