What is the purpose of code signing?

This answer is partially correct in that signing verifies origin, but it omits the vendor's certification of integrity and the assurance that the code is free of malicious modification, making it incomplete.

Code signing allows the vendor (as the code signer) to attach a digital signature to software, certifying both its origin (it was created by that vendor) and its integrity (it has not been maliciously altered since signing). The certificate-based signature provides assurance of authenticity and, by extension, that the code is as the vendor intended - free of unauthorized modifications or malicious injections.

Code signing does not technically guarantee software is free of malicious code on its own; it verifies the identity of the signer and integrity of the code, not a security audit of the code's content - and it also omits origin verification.

Code signing is performed by the vendor/signer as a single entity using their private key; it is not a dual-certification process involving both a separate vendor and a separate signer, making this technically inaccurate.