Secure Boot is ostensibly a security feature, making it impossible to load kernel-level code that hasn't been signed by an authority recognized by the device's hardware. The idea is that this makes it harder for nefarious actors to do nefarious things to your system, because they'll be blocked from the most basic access to the system hardware and be forced through normal OS security channels.

In practice, of course, it's just about solidifying lock-in with a cover story that's obviously weak to those with technical experience but is justifiable to politicians and regulators who would otherwise be all over MS for facilitating this kind of funny business.