The unit is the size, not the garment. A style that sold out in the middle sizes and sat untouched at both ends is a completely different lesson from one that sold evenly, and only counting at size level shows it.
Buying happens far ahead. Orders for a season are placed months before it starts, which means last year's sell-through by size and colour is the only evidence available when committing the money.
Markdowns are where the margin is decided. Knowing what remains, at what age and at what cost, is what turns a reduction into a planned recovery instead of a panic in the last week of the season.
Knowing customers is the independent's advantage. A note that somebody wears a particular size in a particular label turns a new delivery into three phone calls that sell it before it reaches the shop floor.