Well the reason FB/Foursquare/Google etc add these to 3rd party apps is so they can get data. Example if you visit a website which has a Facebook like button, your browser fetches the js files/which maybe even makes an API call to let FB know your IP (and hence location). All this data is fed to the giant system that feeds you ads.
Adding their SDK to other apps/sites (even if there is no user facing need) is a common strategy used by most big companies to get data. In return the app that puts in the SDK gets $ from the company.
You have no idea what you're talking about. Apps use the Facebook library because a good portion of end-users want to be able to login with a Facebook button --or Google, or whatever that doesn't require them to create a user/password account. It's just that simple.
