Because view controllers in iOS carry many responsibilities and are closely related to the app screens, a lot of code ends up being written in them because it’s just easier and faster that way. In the long term, though, all of this code piles up and becomes hard to read, modify, reuse and test. This happens so often that the joke is that MVC is short for “massive view controller.”
Even if you're unfamiliar with the Model-View-Controller pattern, if you have the ambition to develop applications for one of Apple's platforms, you need to learn how views (iOS) and windows (macOS) relate to controllers and which role the model plays in a typical Cocoa application. Fortunately, MVC is easy to learn.