For me It's music discovery. Trying to find a particular net radio manually throughout the web that fills your criteria of music is obviously hard so this extension makes it easier to browse.

Other than that I use the browser for work and leisure so I end up browsing alot, since the browser is on most of the time I can turn on the radio and play faster from the extension instead of browsing to the dedicated website or swap to a different app for the same function. You can make the argument that a bookmark would work just as well but you can do it in the add-on too and make a library list of them that you can press the next button to change to a different one.