As I write, it's June 2018, and Apple's annual developer conference finished just over a week ago.
Apple make changes to iOS and macOS, and developers spend the whole summer working on compatibility updates for their own apps. As well as visible changes to the OS, there are always changes behind the scenes. It's important for developers to keep up with security and new language features that aren't visible to customers. This is part of my normal routine, I know I'm going to be busy from June to October.