Probably to account for people who won’t store it properly, degradation of the packaging material, etc.
For example, if you store your blister pack of KI on a sunny shelf in your bathroom, UV rays eventually weaken the plastic packaging, cracks develop in the plastic letting in water vapor from your shower, and a stray mold space makes its way in as well and eventually you end up with mold growing on your pills. The KI itself may still be perfectly fine and able to do its job, but that mold might make you sick.
There’s a story floating around the internet, I can’t comment on its truthfulness, that 3M accidentally created one in one of their factories back in the 80s.
Allegedly the plant made adhesive tape, and as such had enormous rolls of plastic that were being spooled or unspooled by machines at very high speed, and at one point the right conditions existed for static electricity to build up in such a way that it created a sort of invisible wall that people were unable to pass through.