Also, the web changed; there are webapps that require gigabytes of memory out there, such as Gmail.
All respect to Google, I use a Chromebook for offline writing and general browsing and it's wonderful, but I agree that I'm not seeing the use-case for this one yet.
Maybe they'll port Android Studio to it? That might be pretty nice.
I remember the agony I experienced when I needed to use Word365 online for a project. The javascript implementation was so slow and resource heavy that I was almost going to go crazy. Google Docs on the other hand worked fine most of the time. But none of them could replace a native MS Office or Libreoffice. Maybe with webassembly things are going to get different, but adding abstraction layers come at a cost.
Also these machines need to run videos and process pictures and maybe movies. With those requirements performance doesn't hurt.