No one needs them for discovering new software. You go to the app store to get a specific piece of software you already know about and whose reputation you’ve already verified.
No one needs them for code signing. If you want the real firefox.apk you can download it from https://firefox.com/ and notice a lack of SSL alarms going off. What’s the code signing for — to let me know that HTTPS works?!
All I really want is an OS facility that only executes code that has been installed explicitly by me, and always in a sandbox. That’ll stop that dodgy email from executing r00t.sh via an exploit in my mail client.
App stores and code signing popups are proxies for that kind of OS facility, but I’d rather it was just part of the OS explicitly.
App stores leave me with all sorts of unwanted properties, namely Google / Apple throwing their unwanted opinion around. Search for Instagram in the iOS App Store: the top UI element is “InstaMap”, thanks Apple!