I believe they MIGHT need to be refreshed after a certain amount of shots. Consider the autofocus… It’s like a motor that moves some things around until you see it clearly.
It is also very helpful when buying items on second hand at dealer shops or over eBay.
Last but not least this is a good way of telling if the lens came out on a period that people complain about focus issues and Canon replace them for new ones. Similar to say, a certain car year that sucked for the model or even the whole manufacturer.