Basically it is storage tiering. The operating system and the apps use the faster storage, while for other data, the slower storage is used.
It's similar to PCs a few years ago that featured a SSD (e.g. 256 GB) and a HDD (e.g. 2 TB).
I don't know what google is trying to achive, but it's likely to cut costs and improve battery life. (If my assumption is true that the slower storage is cheaper and uses less energy).
Seems a bit like small cell towers on drones. With few dozen drones, you can likely get a pretty decent range.
It probably only makes sense in military and in desaster relief (e.g. after an earthquake when power and cell towers are interrupted).