Purchasing and production functions coincide because of the fact that the production/preparing of menu items both limits and defines which food items are purchased.
For example, some restaurants near the sea; for instance, various restaurants in Maine may serve fresh clams or lobsters, which were caught that day. Another example of this are the various restaurants in Italy serving freshly caught fish from the sea.
Both of these instances would reduce the amount of frozen fish needed/bought from various food distributors (Clark, Gordon, Sysco) instead, these purchases could be obtained through local markets/docks.
In addition, more inland, where fish would be delivered frozen, chefs/cooks have more time between the purchase and preparation time; since the food is frozen, these menu items do not have to be cooked right away.
Another factor is the price that can be charged; restaurants near the sea are able to charge higher prices; conversely, if restaurants more inland decide to have fresh fish shipped in, they too are able to raise the menu prices for these special occasions.
High quality production of food requires the purchase of high quality food products, proper storage, and food to be held at proper holding times, prior to service.
In addition, geographical location as well as availability may decide which food items are served at restaurants; however, the menu will always decide which food items are purchased from distributors.