I remember about 10 years ago using UPSERT in a Teradata database to do delta-diff updating of tables.
Basically, I was updating a table from an external file, and I wanted to add what was new, leave what was the same, and delete any records no longer in the external table. Using UPSERT to insert new rows, update a timestamp on existing rows, and then afterwards delete any rows who's timestamp was before the UPSERT operation. Because of the nature of the file, doing an external patch/diff approach was difficult, but the UPSERT operation worked like a breeze!
Basically, I was updating a table from an external file, and I wanted to add what was new, leave what was the same, and delete any records no longer in the external table. Using UPSERT to insert new rows, update a timestamp on existing rows, and then afterwards delete any rows who's timestamp was before the UPSERT operation. Because of the nature of the file, doing an external patch/diff approach was difficult, but the UPSERT operation worked like a breeze!