It is not efficient to purchase for one job at a time, especially when multiple jobs have shared demand for raw materials and components.  When you combine your purchasing requirements, you reduce the number of POs to be generated and processed and you can qualify for the best prices.  MRP automatically generates POs when needed based on shared demand among jobs and items where net demand falls below item reorder points.  When items are received, the stock status inquiry clearly shows the jobs that need the item so that material can be received directly to work centers where needed.