It is also about being presentable at your destination, especially if traveling for work. I commonly fly US east coast to Europe flights. They depart in the evening and arrive in the morning thought out these are shorter long haul flights I find showering after the flight to be necessary. At that point I was at work all day the day before going directly to the airport and it has been almost 24 hours without a shower. Also I can change into presentable business attire after the shower.