This lead me into building an open source plugin architecture which allows anyone to bring their favorite communities to Yack.
Plugin architecture, with the help of some really talented engineers helped me quickly build plugins for my favorite communities (Reddit, YouTube, Indie Hackers and Hacker News).
As to answer your question, same argument can be made against Apple's Mail app (which has been a big inspiration for designing Yack). People have multiple email accounts, imagine they all had different UI and you had to have multiple tabs open at all times, switching back and forth, each with different UI/UX. That would suck. Even with a single email address, lots of people, including myself use Apple Mail instead of Gmail's website.
I'm a member of multiple Slack teams, it would suck if each looked different and provided different experience and I'd have to use a web browser to talk to my coworkers. Do you think Slack would take off if it was just a website and didn't provide a desktop experience?
Yack! combines multiple communities into a single, unified desktop experience and takes full advantage of the desktop platform, including native OS level notifications, keyboard shortcuts, and many more.