It is excellent for staging mock tables when testing. For example, if your source tables contain millions of records and you want to test a small subset of data, you can use synonyms to re-direct the source table to a smaller table you control so you can stage various scenarios.
In this way, you can update/delete the data in the mock table without affecting your source table. When you are ready to use the source table, all you need to do is re-direct the synonym.