There was a time when personal computers came with security built into their hardware. For about a decade from 1984 on, virtually every PC featured a key lock. Depending on the particular implementation, locking would prevent powering on the computer, keyboard input, hard drive access, opening the case, or a combination thereof. This video tells the story:
From today’s perspective the key lock looks like a weak if not pointless security mechanism. In the best case it makes tampering with the hardware slightly harder—attackers have to equip themselves with tools and spend some time using them—while not at all addressing all the software vulnerabilities that we care about so much today.
Nevertheless the design made a lot of sense.
First, a keylock is a usable security mechanism. Everyone is familiar with key locks and knows how to use them, no complicated setup is required, and there is little potential for mistakes.
Second, an attacker’s physical access to hardware internals defeats most software security mechanisms. Physical access control is therefore a prerequisite for security against certain threats.
Third, personal computers at that time were not really threatened by online or software attacks, perhaps with the exception of relatively harmless viruses spreading through exchanged floppy disks. Someone tampering with the computer was indeed one of the more realistic threats.
Fourth, seemingly minor security gains can be rather effective when put in context. While forcing attackers to carry tools and use them may not seem like a great complication for them, it may suffice to prevent opportunistic attacks as well as quick serial attacks against multiple consecutive targets.
Security technology has evolved to make key locks obsolete, but they made sense at the time of their introduction.