The primary function would be as an audio interface in order to use it as an output module to connect directly to Ableton and use your DAW to record any audio coming out of your circuit. You should also be able to send audio from your DAW into the LittleBits as well to use the effects, or mix some samples into your patch. The audio quality isn't quite CD quality, but is decent at 44.1 12 bit.
In order to control the circuit from Ableton your going to need a plugin that converts Ableton information into control voltages (CV). There are a handful of plugins that essentially convert information from your DAW into CV and sends it to your audio interface (in this case the USB I/O). You then assign this to an output on your audio interface and then can patch the CV out to an input (for LittleBits the output of the USB I/O audio interface is the out bitsnap which can be connected be the rest of the synth circuit). You can also use these plugins to send CV to your normal audio interface as long as it has an output that is DC coupled. If you have a CV LittleBits you could even use the USB I/O to send CV to a Eurorack modular synth using the plugin you choose.
Another feature of these plugins is that they can receive CV from the USB I/O and convert it into information your DAW can use. Essentially you could set up a little bits hardware controller for your DAW. I haven't expored this yet, but plan on it in the near future ;D
The USB I/O page listes a few plugins you can use: "This allows you to use software programs like Max, PD, and CV Toolkit with w27 USB I/O to control littleBits modules by creating “virtual” littleBits modules like sequencers, low frequency oscillators, and much more. You can also control the aforementioned programs with littleBits modules to create hardware controls for your software". I personally use Expert Sleepers, Silent Way plugin and recommend it especially if you have one of their Eurorack modules. You don't need one of their modules for it to work though I have not tested it out with the LittleBits yet so I would get the free demo to see if it works. I have not tried any of the other plugins listed, but will probably check them out just to see what kind of features they have.
I hope that helps. If you do choose one of the other plugins it would be way cool if you posted whether it work