They are trying to get out of having to fulfill law enforcement requests because they have this data. If it’s only on-device (i.e. no web-based view) then Google can’t give law enforcement data they don’t have.
Eehhhhh. Encryption at rest is technically possible here, no? Though I guess they could still be gagged and coerced by law enforcement to MITM the front end (browser/client). But they could also be coerced to do that in the mobile app too, so nothing seems to have really changed.