It is not uncommon for people to own more than one iOS device. A lot of people who own an iPhone are likely to have an iPad laying around somewhere as well. Users who own multiple devices expect their apps to work well on any screen they have nearby. Ever since Apple launched the iPad, it has encouraged developers to make their apps universal to make sure they run on both the iPhone and the iPad.
A lot has changed since then. Currently, there are more screen sizes to keep in mind than ever before and the tools that you have at your disposal to make beautiful layouts have gotten better and better over the years. You can use a single storyboard file to set up your layout for any screen size that is available to users. The best part is that you don't have to start from scratch setting up your layout for every screen size. You can use a single layout and tweak it as needed for certain screen sizes.