Okay, so here's a scenario for you. I've just built a nice new Ara phone. It has a computing module, a camera module, an LTE/GSM+SIM module, a 802.11a/b/g/n/ac module, 128GBs of storage, a touchscreen and a fingerprint reader.
It's the first time I've put this device together, with brand new parts out of the box. How am I meant to download the drivers? I can't use the WiFi, or the cellular modem, I don't have drivers for them yet. And I can't display any kind of configuration, because the display isn't set up either.
The "phone" has the main OS pre-loaded (I'm presuming a bare kernel on the Computing module, as that's what would decide what version of binaries and the like would be needed), so it can boot, but there's no functionality on it to mount the storage, or bring up the display, or even to start the WiFi and/or Cellular data, because there's no drivers for that yet.
The way I'm suggesting means that all the drivers are compartmentalized and available from first boot. The moment you slot a new module in it's ready to use. And while you can update the software onboard, you don't need to download software to get the system up and running. The on-module flash would also be locked into read-only mode for regular operations, and only unlocked as read/write when an update is required.