Code signing prevents users from downloading and installing unauthorized applications on the device. Apple says the following about it:
"Code Signing is the process by which your compiled iOS application is sealed and identified as yours. Also, iOS devices won't run an application or load a library unless it is signed by a trusted party. To ensure that all apps come from a known and approved source and have not been tampered with, iOS requires that all executable code be signed using an Apple-issued certificate."