Most of the blue mailboxes have single pickup time, which I understand.
However, mailboxes around post office or inside the post office have multiple pickup times listed, such as 7am, 11am, 4:30pm and 6:30pm.

My question is why?

Is it that they get emptied because they're full, and if that's the only reason, why post the intraday times, rather than the last pickup of the day?

Is it to ease people's mind that their mail won't sit in an exposed to weather, theft or hit-n-run (vehicle collision) mailbox all day long?

Is it because the mail is sorted as soon as its picked up and gets to the destination sooner?

I ask because I dropped off a letter on Sunday night into a mailbox that had a 7am pickup time. This letter is going to my local Netflix facility in same town. As of 2330 on Monday night, it hasn't cleared my que.
So if a post office mailbox that gets emptied at 7am can't deliver the letter to a large sorting facility (read: if any business gets mail delivered multiple times a day, Netflix would) in same town, than what's the point of multiple times pickup schedule?

Mario