Do we need an ETL tool? When do we go for the tools in the market? | ETL Testing Interview Questions

ETL Tool:
It is used to Extract(E) data from multiple source systems(like RDBMS, Flat files, Mainframes, SAP, XML etc) transform(T) them based on Business requirements and Load(L) in target locations.(like tables, files etc).
Need of ETL Tool:
An ETL tool is typically required when data scattered across different systems.(like RDBMS, Flat files, Mainframes, SAP, XML etc).