Joining data together is necessary when you have a standalone table with no address or x,y data. Joining can be performed on under the following scenarios:
In order to join data together, you need to have the standalone table and a shapefile. Both files need to have a common geographic identifier so the table and shapefile can be joined. For instance, the following dataset was downloaded from the NHGIS and there is a field labeled GISJOIN in both the table and the shapefile.