See, "apps" made sense in the prehistoric times when single-threaded desktops with limited RAM were primarily used to do one thing at a time. But things changed. The concept of an "app" have not been making sense for a long time: you can run Gmail and Facebook in two browser windows (which together live in the same process) or you can have a single window (Chrome) launching one process per tab, or you can have a terminal with several processes running in different windows, or you can have a copy of Microsoft Word embedded inside of an Excel spreadsheet. Similarly you can have an iframe with embedded Google Maps inside of your Wordpress blog. So what's an "app" then?
Microsoft correctly recognized in early 90s that the concept of a "program" or an "app" was obsolete, and modern UI should be using "window", "document" or "folder". An app is just an implementation detail: you can have 3 apps involved in rendering a single document or drawing the same window. Early smartphones couldn't handle these tricks (like multi-tasking) well, so this temporarily reincarnated idea of the "app" came back from the grave, to be happily copied by Unity just because "iOS has them".
Boo.