It solves two problems: one is someone covertly or foolishly plugging in an untrusted USB device (which might be easily missed on, say, the back of a desktop) and it means that checking to make sure that only a keyboard or mouse are attached is as simple as putting tamper-evident seals on those cables.

Attempting to authenticate USB devices is a very hard problem — a sufficiently advanced attacker can spoof manufacturer and device IDs, even if you lock things down to prevent anything other than a keyboard or mouse it's possible to send keystrokes to open the wrong website, there's always a chance of an exploitable flaw in your USB stack, etc. — but anyone diligent can be paid to walk around every week checking to make sure that a seal is solid and the tamper-evident stickers have the same serial number as listed on the inventory. There is a real value in having things where the failure modes are obvious and intuitive.