Warehouses do two things: they store items, and they increase delivery range. Most structures can only deliver items to 10 tiles away or so. Warehouses have a practically unlimited delivery range.
The wheelbarrow people technically exist for each city (not villages) in the world. Their overhead is incredibly small, though, as it's essentially moving along a number line. I believe that the current performance drag is down to logic running on individual structures, but I'll look into that more after tactical combat.