The only surprising thing about Spotify launching a web app is why on Earth it took them so long. A desktop app is a nice bonus, but the web app should be the focus, especially for a cloud service.
Or maybe that's just what everyone else seems to think, MOG, Rdio, Deezer and so on.
It also makes sense, a web app, even as an alternative to the desktop app, is not only a smart move, it's a necessary one.
The web app means users will be able to follow a link and start listening to the song immediately, without having to start the desktop app, go out of their browser and then back again to do it.
It also means Spotify will be able to offer better embedding options. Currently, users can embed playlists or songs along with a Play Now button, but those send users to the desktop app. If you're not a Spotify user, you're out of luck.
With a web app, Spotify could offer 30-second song previews, like others do, enabling non-users to get a taste of both the song and Spotify.
Anything that Spotify can do to increase user numbers should be pursued relentlessly if the company ever wants to turn a profit while still sending most of its revenue to copyright holders.
The site should be rolling out over the next few weeks. Users will be notified of the new option when it's available.
Spotify still expects most people to use the desktop app which offers offline playback and a few other features which won't be available on the site. As a bonus, it means that Linux users will finally be able to use Spotify.