Improvements in digital audio technology now give us the opportunity to take a quick and simple shortcut to adding a harmony track at any time!
Either because they (or the lead vocalist) weren't skilled enough to sing in harmony, or because the harmony wasn't thought of until after the recording was done - increasing costs to call back the vocalist and re-record the harmony track. Traditionally however, this wasn't always a straight forward option for the average songwriter. Adding a vocal track to harmonize with the lead vocal is a simple way to add depth to any recording.
