Pages Router matters because modern web apps break when server/client boundaries, caching, rendering, or state are misunderstood. It gives builders a precise word for the thing they are changing, debugging, reviewing, or shipping.
ELI5
Think of Pages Router as a named part of the app screen or page machine. It gives one small job a clear name so the whole app is easier to understand.
In practice
Use it when deciding where state lives, what renders on the server, what hydrates in the browser, and what gets cached. In practice, name the owner, input, output, failure mode, and proof before relying on Pages Router.