The single page aspect is mostly just for 'slickness' - content is displayed instantly, animations are used and the page only needs to be loaded once. It is becoming more popular among the web; many portfolio sites that I come across are on a single page. The portfolio, business card, resume, etc... type site is well suited for a single page since they are simple, slickness is fairly important and search engine rankings aren't as important.

A side benefit of Pagify is that it DRYs up what would otherwise be duplicated elements in content sites with static HTML pages, i.e. header, navigation and footer - and without any serverside processing.

That being said, a single page and this plugin in particular, certainly aren't the best option for all sites. The reason I made this was to make the development cost virtually the same for developing single page sites.