Power BI enables users to connect to lots of data sources and manually publish/ upload the data to the Power BI service. However, the manual data upload would be tedious and inefficient every time the data changes, hence, the need to set up a scheduled refresh that would connect whatever data sources you set up and automatically publish the data into the Power BI service. This also provides a way to connect the Service (the Power BI online capabilities) with any on-premises data sources (data sources that aren’t in the cloud) that you have got : Excel files, Access database, SQL Server, Oracle databases etc.. These functionalities are achieved by the Power BI on-premises gateway.