Hi Hemant,
You have totally misunderstood it.Planning is not related to performance and tuning its a different topic altogether.
Basically planning is done for budgeting and forecasting of business.
Planning cubes have read and write facility where in if user enters any value then it will be written directly into the cube.
Check these documents related to planning.
http://scn.sap.com/docs/DOC-8265
Hope this helps.
Regards,
AL