The "URL to page/external address" is a property type you can use any way you like. It lets the user select a page, document etc. What's selected is stored as an url (could be a permanent link type url if it is an internal object). You can use the data stored any way you like, for example to present a link.

The Shourtcut/external link selector is a special "property" you use when you want to replace the whole page. Episerver will redirect to the selected page (or in the case of fetch data, it will give you the properties from a different page).