Most ships sail at six. Oceania often does not, and that single difference changes the character of a port. The coach parties leave, the streets empty out, and the place goes back to being itself.
It is the hour when a market packs up and the restaurants fill, when you can sit down to dinner ashore instead of hurrying back for it. On the overnight calls you get the morning as well, before anyone else has arrived.
The ship is still there when you want it. That is rather the point.