Question

why we use secondary memory if we already have primary memory (class9 level)

Answer 1

Secondary memory is slower than primary memory but can store and retain data, even if the computer is not connected to electrical power. It also has substantial storage capacities, ranging from megabytes to several terabytes of storage space within single memory.

Answer 2

Primary memory is faster than Secondary memory but we need secondary memory because it's size is far more larger than Primary memory.
Secondary Memory is used to store data that is not in use right now but will be used later.
For ex. You have 4 games stored in your Secondary Memory but you can only play 1 at a time using Primary Memory. That's the use of both the memories.