IIRC you have to have the phone in question registered as a developer phone to do that. There are sites that will add your phone to their dev account for ~$10 (vs the $100/yr dev program) but it's just a barrier to entry that's a little too high for the average person. Whereas 5 minutes with a one click jailbreak tool + Cydia can make the process much easier and you get app updates (it's been a while since I did iOS work and as I was leaving the scene things like test flight were making it even easier to distribute dev/test/beta copies of an app and even offer a way to update them so my last comment might be out of date).