The software and dashboard that was shipped with the Chromecast was from the same company as the original version of the TV. As long as you're a one-person household, a new launcher that puts personalization and smart recommendations at the forefront, as long as you're a one-person household, is the latest attempt at putting its services in front of your eyeballs. Google promised to support more profiles after adding kid profiles so you don't have to listen to endless Baby Shark recommendations. It has been a long time since that feature was available.
A server-side update that is live should allow users to add multiple user profiles to their devices, though it may still require a restart or reinstallation of the app. I can confirm that the option for another account to be added was given to me when I started playing with my Chromecast again.
You will be able to personalize your interface with watchlists and smart recommendations based on your viewing habits once you add more accounts. This will hopefully help cut down on the amount of time it takes to decide what you want to watch, as well as the amount of time it takes to recommend something to your partner.