Ignoring any additional processing that might be used, winemakers could empty the barrels into bottles and sell their wine. However, during the process of barrel aging, the smaller containers may develop differences. So the winemaker will likely 'blend' wine from different barrels to achieve a uniform result. Also, the winemaker may blend together different varieties of grapes to achieve desired characteristics.
For example, blending a little Merlot into a Cabernet Sauvignon can give it a more "mellow" taste.