Unless you’re the type of person who likes to write everything themselves, chances are you’ll have a lot of dependencies in your code on classes from external frameworks. These don’t often fit within your personal (or company’s) API guidelines or architecture. You also can’t apply dependency inversion as easily as with your own classes.
Since in Swift you can extend any type (including types from other frameworks) with whichever methods or computed variables you please, controlling external types is easy.
The approach I use for this is wrapping the external classes into a protocol I control. It boils down to extending the external class to conform to one of your protocols.