Hello Bluewatercat,
The side brush module is needed for edge cleaning or for chair legs for example, Roomba always cleans the edges on the side of the side brush. The double brushes/rubber slats are responsible for cleaning the floor.
Best regards from your iRobot Team

Roomba, always runs along the right side of the wall. As far as I can see, it only needs the rotating brush for the edges.

Only the manufacturer will probably be able to answer this question :)
But it works very well! by the way, the street cleaning machines also have it like this :)