Jailbroken devices can attack application logic to cheat in multiplayer games, somewhat attack DRM systems for video content (though Fairplay isn't especially vulnerable here), gain access to chargeable features without paying for them (decompiled Spotify APKs that do not feature advertising without having to pay exist on Android and are a non-trivial revenue risk) etc etc.

In more open systems you are usually more able to run detection software for the above without sandboxing.