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I dislike frame bags for how they sometimes bulge if not packed perfectly.
I also find when doing more distance I prefer my knees to be following a comfortable path and not trying to avoid rubbing on a bag.
And I like stuff sacks for how they compress things.
A bar bag and stuff sack is already a nice thing... I just want similar on the rear of the bike. And it makes sense to point it in the direction of travel and put it on top of a rack rather than hang a cone-like shape under the saddle (which is what the Apidura and others are today).
Full size frame bag and bar bag