In my view it is critical in a manufacturing environment.
The reason is that there may be purchases driven from two levels:
1). The Master Production Schedule, MPS, may drive orders for purchases of high-level bought-in items required in a configure-to-order or build-to-order environment.
For example my order for a 6 litre V12 Audi R8 diesel may include the Bose Aqousitmass sound-system option. In order to build my car in time, the sound system has to be ordered from Bose to be delivered in time to be fitted to the car before the interior trimming is complete and the dashboard is installed.
2). At the Material Requirements Planning, MRP, level, as well as generating new production and purchase orders, there may be re-schedules generated for existing purchase orders, which may have timing or quantity changes which the buyer needs to communicate to sub-contractors or component suppliers.
In my silly example above, if I can't arrange the finance for my R8 for another six weeks, the purchase order for the full leather interior gets re-scheduled and the twenty cows on the farm in China get to live a while longer.
The manufacturing environment is dynamic with companies no longer interested in building expensive products to sit in warehouses, so buyers need to be clued in to customers' changing requirements as expressed in the schedules at various levels.
HTH.