It's also timed to allow business travelers a full day on both ends. A midnight or late-night departure allows a full work day before the flight, and even allows many connections to have a full day before taking a late-evening connecting flight to ATL. This, combined with the early arrival allows for a full business day in Seoul upon on arrival, or a mostly full day upon arrival even if taking a connection onward from ICN. The SEA-HKG flight only allows about a half-day in SEA, and less of a business day if originating from elsewhere and having to connect through SEA.