International flights are usually far too cold to endure without a jumper, fleece or some other warm, long sleeved item of clothing. Even more necessary when travelling in Europe's colder seasons.
When landing at LHR in winter , I often see people departing the airport in shorts and t-shirts when I am wearing a warm jacket that I have unpacked from my suitcase. Maybe I have become too acclimatised to Thailand's hot weather.
When I see people travelling in economy, wearing a shirt, tie and jacket for what is around a twelve hour flight, I often wonder if they are hoping to land an upgrade if available. I suppose there must be some advantage over the guy in the wife beater.