If you have a 16 GB iPhone or iPad, you quickly come to regret it, as app sizes have gotten bigger and bigger over time. This is thinks in large part to apps needing Retina-sized assets, and apps that require separate 32-bit and 64-bit code. The new asset sizes start to add up. But a new iOS 9 feature, app thinning, could help out a lot with that, as argued by Ars Technica. Now an app can download just the set of code it needs, just the size of images it needs, and just the GPU code, whether it be OpenGL or Metal, that is required.
Even if you have a 128 GB iPhone 6 Plus, this will still benefit you. App downloads and installs will be smaller as they will now only need to download and store the exact files they need, provided the developer uses app thinning properly. This means you can install more apps, and use up less cellular data when on the go. This also makes the entry-level iPhones and iPads more valuable. Though, I still would like to see Apple introduce at least 32 GB as the new entry-level storage size, especially as flash memory has gotten cheaper and chaper.