Google is in the business of collecting data. Data from everywhere it can get it's hands on, and as much as possible. In a similar fashion to this latest privacy violation, a few years back google got it's hand slapped for using it's street view cars to also collect data off of unsecured wireless networks. I don't know what their intentions were, but there's no reason for them to have it. They probably don't even know what to do with it yet. They'll figure that out later.
That is just them taking information from us without our permission. Google already gets lots of data from us whenever we use it's products. They get data and we get services in return. After they get the data they analyze it.
For some known examples:
If you use google search they know your search habits and interests. Why? Well, to show you ads that appeal to you is one reason.
If you have an android phone and you use google maps they know where you are and where you go. One use of that is so google maps can have an idea of the traffic speed and find a faster route around slow areas.
Why they would want to listen in on you at you pc? Don't know, but they also has all sorts of data from you that we trust them not to inspect. Gmail is one example. Laws keep them from using some data, and even forbids even collecting it, but that doesn't mean they don't have it somewhere.
Too bad this whole data collection is becoming more and more common. Governments do it, but lack the efficiency that a private company can have. Even Facebook, Apple and Microsoft are data collectors. We aren't as anonymous as we once were.