Rahul,
To add to the other expert's excellent answers, let me also share a business philosophy for allowing start dates in the past.
It allows a planner to zero in on his worst planning problems immediately; under a scenario with many levels of the BOM; by running a single report of planned orders with start dates in the past for the lowest level of the BOMs. The 'oldest' dates here represent the requirement against the 0 level products that will be the most delinquent. In many companies, these are the issues that a planner is required to attack first.
This prioritization technique is particularly useful when the planning situation is so bad that a planner cannot possibly cycle through the entire MD06 list between planning runs.
Best Regards,
DB49