Secure Boot is the first component in a verified boot chain from initial power-on to application level code. Signed, verified firmware boots signed, verified kernel with strict authenticity and integrity guarantees. The goal is, presumably, to attest to the authenticity and integrity of everything the system runs, but when it comes to kernel modules and device drivers, userland OS components, and applications, those are the kernel's responsibility. But Secure Boot is an essential link in this chain.