Preventing unsigned code from running, and choosing which sources of software you do want to trust, is not just for DRM. It severely reduces the attack surface

Preventing unsigned code from running, and choosing which sources of software you do want to trust, is not just for DRM. It severely reduces the attack surface of a device and makes it much harder for user error to cause a compromise. Accidentally getting compromised is possible for advanced users, let alone the most tech-illiterate person in the company. iOS being totally locked down in this way is no small part of why exploits for that platform sell for so much more money than exploits for other platforms. Of course, it is also used for DRM on that platform.