I do this all the time when there’s not a shortage of seat…
2 reasons :
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When you have a ton of books/papers inside, a bag can be really heavy… and when I’m on a two-hours long ride, as there’s not enough space to put my bag on the floor or underneath my seat, I have to put it on my legs… They become numb
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Keeps the creeps away as @framos1792 said.