Is it necessary every time to delete Plan before running master scheduling?
The system does it automatically unless the planned line status is approved - why do you ask. The answer technically is yes because of the entire inter-related nature of the plan and the fact it will have moved since the last run, so any unapproved planned order has not been actioned so should be recreated if needed.
It depends also of your necessity of running it, when doing it you will also have 3 options given by AX:
Regeneration: As the name says deletes everything (except status=approved orders) and creates everything again.
Net change: Only for a dynamic master plan, will generate orders just for the changes from the last master plan execution. And reevaluates action messages and futures dates for everything.
Net change minimized: Only for a dynamic master plan, will generate orders, reevaluates action messages and futures dates, just for the changes from the last master plan execution.