This is also a best practice for defining security settings in SPO.
Why is this important?
Knowing how the synchronization happens helps for building scenarios with user information like f.i. business units, locations, department or similar important user data for SPO or Apps using AAD or SPO user profiles.
Since some user properties are automatically synced company specific user information should be put into these synced fields – even if their names maybe are not fitting perfectly, but that´s the “built-in way” without any apps doing sync-work.
So that´s what we often recommend because this is simple to configure in your own DirSync.
Here are some samples for mapping user information into existing fields: