As someone who's rather new to Mumsnet, but who has done a fair amount of flying, I think that's a very valid question, actually.
I used to travel very light (carry-on luggage only), on a tight schedule, to professional events all around the world (often in non-English speaking countries)
I ended up always wearing the exact same set of clothes on a flight. Something comfortable and practical that would last me through the whole "day" (sometimes over 30 hours due to time zones!), yet would look decent enough as I'd navigate within and between airports (in particular, security and passport control, and endless corridors. You want durable shoes that are easy to put on and off), through public transport, and to the (sometimes rather posh) hotel. You want to not look entirely like a tramp when trying to check in an immaculate 5 star hotel in a foreign place with conservative cultural norms (you never know for sure if the booking went through OK till you're standing at the reception desk, or whether you'll have to get your credit card out there and then, and hope you don't look so dodgy they refuse service). Consider these will also be clothes you may have to wear in busy foreign streets before reaching your destination (if like me once, you couldn't claim taxis on expenses, and had to leave the airport by bus/foot), and shouldn't scream "mug/rape me" either.
Obviously my best clothes would be tightly packed in my carry-on, to have something nice and fresh to wear at the business event. But what to wear on and around the flight did require some thought (especially since they were clothes I may have to sleep in, if one of my flights got delayed and I had to spend the night in an airport!)
Obviously no such preps are required for a short holiday trip to the Mediterranean.
But good travel clothes is something any seasoned traveller will carefully plan in advance. Both in term of form (looking good is always a plus) and function (e.g.: do they dry overnight with minimal wrinkling after being washed in a hotel bathroom sink?)