We have been contracted by a large group to build a DWH for them because we already have ETLs for 4 of the 6 systems they need to integrate.
Instead of working with external BUs or external connections, it would be handier to import the entire BU of an existing ETL into the new project. This would allow for cleaner development as External BUs and External Connections are very limited in terms of adding them in the project tree and working with them.
We would then be able to still develop the Master ETL for a certain system and import new logic quickly as a new BU or a replacement for an existing BU.
The customer will also be very happy as he then would be up & running much faster.
For reference (in terms of modular development in general):