Leather arrives by skinning, so its supply tracks how many players are leveling through skinnable zones at any moment. Leather — one of the subtypes World of Warcraft files under Tradeskill — gets this table as a focused view: the same drill-down the in-game Auction House browser applies when you filter by subcategory.
Because this table is filtered to Leather only, it stays short even when the parent category is large — a fast way to scan a single subtype without paging through unrelated items.
The "Browse subcategories" links on the parent Tradeskill page list every other subtype if you want to compare markets side by side.