The reason for this is that the beta channel is better tested than the developer channel, so if you want to try out new features with the reassurance that it will be relatively stable, the beta channel version is the way to go. And Google has some new "tricks" it wants users to try out.
According to Google's blog post:
The best thing about this new beta is speed — it's 25% faster on our V8 benchmark and 35% faster on the Sunspider benchmark than the current stable channel version and almost twice as fast when compared to our original beta version.Check out the new tab dragging feature below. And you can download the latest beta here.
Some of the new features you can try out include form autofill, full page zoom and autoscroll (both thanks to a new version of WebKit), and a cool new way to drag tabs out to get a side-by-side view (shown below).