They are indeed for modularity. One benefit here is that dealerships can quickly swap part of a battery (cells and controller) to make repairs quicker. The other benefit is that smaller batteries can be made by using the same components and code.

Most modern cars will have lots of separate controllers. Many of the functions of the car are largely independent and having them on separate boards means they can be physically closer to the things they are controlling, and the number of functions that each is responsible for is reduced which simplifies design, coding and test.

I'm going to guess "modularity". Probably also cheaper to shoehorn more smaller boards into the car, than one large board.

Anyway, as for connecting them and coding the interfaces, they've been doing that for ages. Have you heard of CAN bus? It's actually pretty cool.

That's pretty interesting, thanks for sharing. Hadn't heard of CAN before.