Then what's the purpose of signing in the first place?

The keys for official releases are supposed to be kept by a trustworthy senior management or some people with highly restricted privilege. Also the software/firmware shipping process must be well audited if there are serious (not just pretending to be) security requirements. The keys should not be exposed to a random technician or a new hire who can possibly responsible for destroying your and your clients' businesses. It looks like they didn't have a quite "formal" shipping flow at least in this case.