A designer handbag holds its shape and resale value based on structure, so an empty, uncollapsed bag develops creases and dents that are hard to reverse. Stuffing and upright packing is the entire technique.
Pro tip: If you no longer have the original dust bag, a soft pillowcase works as a reasonable substitute for the move.
Watch out: An unstuffed handbag packed empty can develop permanent creases and a collapsed shape that lowers its resale value.