Some of the elm-pages APIs will take internal URLs and ensure that they have the canonicalSiteUrl prepended.
That's the purpose for this type. If you have an external URL, like Pages.Url.external "https://google.com",
then the canonicalUrl will not be prepended when it is used in a head tag.
If you refer to a local page, like Route.Index |> Route.toPath |> Pages.Url.fromPath, or Pages.Url.fromPath