I'd long suspected that disposable ballpoint pens had holes in the caps so that they'd dry out faster. It's planned obsolescence.
But that was incorrect. It's actually to save lives.
Every year, about 100 people in the USA choke to death on pen caps. The purpose of the hole is to provide an air passage in the event that the pen cap becomes lodged in an airway. Science Alert explains:
By simply adding a larger hole to the top of the lid, pen company BIC increased air flow and the chance that people would still be able to breathe even if that happens - a design feature that, quite literally, saves lives.
"In addition to help prevent the pen from leaking, all our BIC caps comply with international safety standards that attempt to minimise the risk of children accidentally inhaling pen caps," the company writes on its website. "Some of these vented caps, like that used for the BIC Cristal, has a little hole in the top to comply with the existing safety standards."
-via Glenn Reynolds