Block jailbreak / debugging execution
If a malware (or something else) want to attack the app, it will run the app "out of sandbox".
So, all the app runtime will be exposed.
Input can be sent to the user, from local notifications or anything like this.
And, because runtime is not secured (cf. #2 (closed)), attacker can also access to network stack, bluetooth stack, storage.
So, jailbreak MUST be detected and blocked.
To test the security of your app, you can use unc0ver jailbreak and run any app with cycript.