On the left is a screenshot of an ad that popped up after the initial "PANDORA" screen during the start up of the app. The screenshot on the right is an ad that popped up during use-time of the app. In order for the company to provide free music for their listeners, it heavily relies on advertising to fund their services (along with the Pandora One service that eliminates ads by asking users to pay every so often to use their product).
Not every app has an obnoxiously large ad space. Some rely on smaller banners or sponsored posts that end up in feeds. A few feature special add-ons that are based on/designed by the sponsor(s). Pandora also relies on audio and/or video ads they sprinkle throughout the use-time of their app (and website service).
These are screenshots of the loading screen, the simpler name for "Intertsitial Screen", that pops up for the app LINE PLAY. The app itself is pretty memory heavy, so it usually needs the loading screen when starting up. This prevents the user from clicking anywhere else on the screen to stop the loading process... Well, save for the Home, Back, Options, and Search keys on an Android phone like my Droid 4. LINE PLAY also uses it as an opportunity to share tips on how to play the game, as shown in the screenshot on the right, and further social interactions between players in the game.