this is something people have done since neanderthal days, i guess, but the internet has made it a "thing". more or less to avoid check-in and even overhead bins, and the xingyiquan piquan you have to restrain yourself from using when morons want to fight for space, you stuff your minimal stuff (maybe a week's worth of clothes at most and do laundry wherever you go) into your backpack that can fit under the airplane seat or is otherwise small enough to carry w/o making you look like a "hiker", and there you go. happy traveler.
whether you've heard of it or not, is that how you tend to travel? i used to use a backpack and a rollerboard, but more and more with various types of karens and their male counterparts flying, i'm like fuck it, you win with your 900L duffel. my bag fits all my not-over-packed shit and fits under the seat. good to go.