It's a good question really, even I wondered this multiple occasions as a child and what I came up to was this.
A pen top essentially effectively protects yourself from the nib and everything else from ink. Additionally some accept that it prevents the nib from exposure to environment with the goal that it doesn't get dry, however then for what reason would some pen tops have openings at the top?
Most presumably, it makes a vacuum inside the top once in a while making it excessively tough for you to open it.
Yet in addition initially the opening on a top of a pen was simply the plan to prevent from suffocation and shocking, if you breathe in it unknowingly and it gets struck, cos as many as 100's of children died in a year due to inhalling pen caps.
This was all I know about pen caps... Do comments if you got some query.
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