Sitting on a motorcycle with stuffed pockets is very uncomfortable. That's the first reason. Wearing leathers without accessible pockets is the second reason. When on the road, you need to bring your smartphone, ID card, driver's license, motorcycle papers and a bit of coin with you. There you go, three reasons.
Three good reasons to use a hip bag or leg bag when you're on the bike. We find they often fill most of our luggage requirements.