It's because they reused some code to grab the MAC addresses, which was also grabbing packets (which it shouldn't have).

They weren't specifically collecting this information. They were collecting packets that happened to come their way. Combine enough packets, and in some cases you'll find emails and URLs and password.

Story says "Google collected the data as a result of code integrated into the software used to identify and map WiFi signals. Google says the code was developed in 2006 by an engineer working on an experimental WiFi project that sampled all categories of publicly broadcast WiFi data. That coded ended up on the systems that power the data collection of the Street View cars."

Because the data is there. If you're collecting it without filtering, you don't have a choice.

I'm speculating, but I suppose value from this data can be anything from geographically locating ip-addresses to improve location based services to wireless-router manufacturer statistics.