Explain the difference between MRP & MPS. Though both components gives you the requirement list, what we gain out of MPS run rather than running MRP. What is the main idea behind this?

The following might help in explaining the difference between MPS and its counter part MRP.

Master Production Schedule (MPS) : 
MPS operates within only one level of the BOM, While MRP can be utilized throughout all   levels of a material’s BOM.  If a MPS is run on a material, the necessary orders are planned at that level. Dependent requirements (if any) are placed on the next BOM level down, and then the process stops.

Main Idea : Master production scheduling (MPS) is a form of MRP that concentrates  planning on the parts or products that have the great influence on company profits or which dominate the entire production process by taking critical resources. These items are marked as ‘A’ parts (MPS items) and are planned with extra attention. These items are selected for a separate MPS run that takes place before the MRP run.  The MPS run is conducted without a BOM explosion so that the MRP controller can ensure that the Master schedule items (MSI) are correctly planned before the detailed MRP run takes place. 

The master production schedule is a line on the master schedule grid that reflects the anticipated build schedule for those items assigned to the master scheduler. The master scheduler maintains this schedule, and in turn, it becomes a set of planning numbers that drives material requirements planning. It represents what the company plans to produce expressed in specific configurations, quantities, and dates. The master production schedule is not a sales item forecast that represents a statement of demand. The master production schedule must take into account the forecast, the production plan, and other important considerations such as backlog, availability of material, availability of capacity, and management policies and goals. Syn: master schedule. 

Material Requirements Planning (MRP) : 
A set of techniques that uses bill of material data, inventory data, and the master production schedule to calculate requirements for materials. It makes recommendations to release replenishment orders for material. Further, because it is time-phased, it makes recommendations to reschedule open orders when due dates and need dates are not in phase. Time-phased MRP begins with the items listed on the MPS and determines 

(1) the quantity of all components and materials required to fabricate those items and 

(2) the date that the components and material are required. Time-phased MRP is accomplished by exploding the bill of material, adjusting for inventory quantities on hand or on order, and offsetting the net requirements by the appropriate lead times.