Taking better advantage of a display is nice but imo the really exciting part of desktop mode is the planned integration with Google's Linux Terminal app (i.e. 1st party linux VM support). I have a Samsung DeX device and while you can get a basic dev environment working easily it can be really cumbersome to make it comfortable to use and integrate with your normal tablet workflow. Being able to install full-fat linux apps and run them in a window would be a complete game changer.
source for planned integration: https://issuetracker.google.com/issues/392521081?utm_source=...