For example, through TweakBox, AppValley, or even some website, we can install some "mod" or "modified games" (such as a Pacman that never dies, or a game that requires your location but the app can emulate the GPS location).
We have to go to Settings and "trust" the app. If we trust the app, to what extend of risks are we exposed to? I don't believe the app can go to the Mail or Gmail app and look at our emails, for example. The only I can think of is that it fakes a log in screen to your iTunes account and lead you to type in the user name and password (Trojan horse), so I will always cancel such log in popup and go to iTunes and log in there myself.