Staging and Operational Data Store
What are Staging and Operational Data Stores and what is their difference?
An operational data store (or "ODS") is a database designed to integrate data from multiple sources to make analysis and reporting easier.

Staging areas are used in databases as a place to store temporary data for import. By filling this staging tables with import data you can check and alter this data before committing it to the final production datatables.

I think previous answers are perfectly valid

Why does one need staging and why does one need ODS?

Check above answer.

What will happen if they are not there?
Answer lies above.