A huge percentage of chicken owners THINK their run is predatorproof.
Only a tiny percentage of them are right.
Go browse the "Predators and Pests" section of this forum if you don't believe me. See how many people post threads along the llines of "something ate all my chickens last night, I though my run was totally safe but it wasn't, I'm posting this as a warning to others".
Just because you have left the popdoor open for <however long> and nothing has happened yet doesn't mean it is a predatorproof run. The operative word there is YET. You do not know which will be the day that something decides to TRY to get in.
So that is why most people lock them in the coop at night. To be relatively assured of having them still be there in the morning
When you let them out, you do not normally shut the popdoor. First, because you WANT them to be able to wander back in to use the nestboxes (much better than playing 'easter egg hunt' all over the yard every day
), and second because they may NEED to get back in, for shade, to get away from another chicken harassing them, for any drink/feed you may have in the coop, or to get away from a hawk or other predator.
The only time people usually shut them OUT of the coop is if you are doing cleaning or painting or such in there, and don't want them mucking around. Or if it is their first time in the great outdoors and they keep running back in and you want them to just get some experience being out there darnit so they'll get used to it
Good luck, have fun,
Pat