what is staging are why we need it

It is a automated process of building a data warehouse which involves taking the data from disparate source systems, converting them into a consistent form that can be loaded into the warehouse and performing quality checks while building the data warehouse.
It typically accounts for 70-80% of the effort in a data warehousing initiative.

If you store data in data warehouse directly it is very difficult to retrieve data from that because various types of database are going to store in warehouse.

So for better performance we are going to store data in staging in common format.