Why put a vent hole in the cap?
Think about the air when capping a pen when the inner cap seals to the section. As you are threading the cap on, the air compresses and needs somewhere to go. If your threads are nice and snug, it may not be able to escape around the threads so it could be forced through the nib to the ink supply. When uncapping a pen, you can have vacuum and it can suck the air (and ink) back out.
With modern feeds the issue may not be as noticeable as it was with older style ebonite feeds, but with every pen being a little different, threads being tighter on some pens than others, some pens will still have ink pulled into the cap.
The vent hole goes above the threads, but below where the section lip would be.