Why did we extend the class Page

Great question actually! It's mostly a matter of preference on how to organize code into groups of functionality. You can put the functions in the original type declaration, absolutely nothing wrong with that. But some people (me included) prefer to create small extensions to group functionality, particularly if that functionality is defined by a protocol.

For example, let's say you had a class, User, that conforms to several protocols like PersonType, AdministratorType, Billable, JSONDecodable, Archivable and Unarchivable.

If you added protocol conformance for all these protocols and put all the methods in the original declaration, for starters your type declaration would look like this:

class User: PersonType, AdministratorType, Billable, JSONDecodable, Archivable, Unarchivable {}

It'd be really difficult to understand which code in User provides implementation for which protocol. On the other hand if we used an extension to group the functionality, it makes our code much more readable. The original type, with information that defines it's identity, exists on its own.

All other code related to protocol implementation would go into its own extension. For ex:

extension User: PersonType {}

extension User: Billable {}

It's purely a matter of keeping your code more readable and organized.