The move is a bid to make Google Play Music more appealing by adding in Songza's top-notch playlist curation. Songza doesn't have a huge number of listeners: less than 6 million, a fraction of the tens of millions claimed by Spotify and Pandora. Google doesn't need Songza for its licensed music catalog or streaming service (it already has a bigger catalog in Google Play Music).
What Songza brings to the table is smart, contextual playlist curation. Currently, you can tell Google to "play some music" and Google Play Music will spin up a radio station of streaming music based on your tastes. It's not really very good at that, and it's not very smart or contextual.
Songza lets you play "music for a Tuesday afternoon" or "music to play at work" or "music to play while working out." It doesn't take a big leap of imagination to see how that fits naturally into Google's contextual search engine. Already you can ask Google questions on your phone and it will provide an answer taking into account your location, time of day, weather, and search history. With Songza, "OK Google...play some music" could automatically know you're at the gym, or work, or that it's a rainy Saturday afternoon.
According to the Google Play page on Google+, there are no immediate changes planned for the Songza service. So if you're a fan, don't worry, it'll continue to work the way it does...for now. In addition, Google says, "We'll also look for opportunities to bring their great work to the music experience on YouTube and other Google products." With Apple acquiring Beats, this is probably a smart acquisition for Google.