There needs to be a simple (and lightweight) way of getting information on a page, that is easy to parse and only provides key info. Using query with prop=info provides some information, but it is more difficult to parse (structure changes depending on whether it's interwiki etc) and provides information that's not needed for just basic checks.
I mainly wrote this for use in bug 11 or related inter-wiki access, where you only want basic stuff for crafting links, with the URLs for both blue and red links.
This patch adds action=pagestatus, which gives info like whether it exists, whether it's a redirect or interwiki, whether it's readable to the user, and the view and edit URLs. It will follow one layer of interwiki links, but not two (e.g. wikipedia:de:)