There are a lot of apps out there that work great on the latest devices, however when you take these back to older models some of these apps are just horrible. It becomes clear that these apps have been built on the latest and greatest, meaning that they may not work so well on less powerful devices.
When developing iOS apps I do most of my dev work on an iPhone 3GS. Doing it on such an old device is extremely beneficial to the development process. If I can get an app to work fast and smoothly on the 3GS it’s also going to work very well on the models after it. Doing it on the latest and greatest and then testing on one of these legacy devices at the very end will be problematic. You will most likely find yourself going through a lot of things you’ve already built and tested just to bring it back up to scratch.
Of course my example is for iOS apps, but this extends from mobile development to desktop too.