The partnership is somewhat surprising, as Waze is owned by Google, which has its own Play music-streaming service that competes with Spotify. However, Spotify's 50 million-strong subscriber base dwarfs Google Play (and every other music service), so it could be a way for Google to prod all those users into trying the Waze platform.
That would have a couple of benefits for Google: It would improve Waze's crowd-sourced traffic accuracy, and possibly drive Spotify subscriber's to Waze's new ride-sharing platform. That service has a strong need for more users, especially since Google is planning to expand it to other cities beyond the Bay Area. For Spotify, which has been pursuing off-beat partnerships (like one with the New York Times), it could further bolster its user ranks.
Whatever the reason, the feature should be handy, especially for Spotify subscribers who already using Waze. The feature is rolling out to Android users around the world (not iOS for now) over the next few weeks, so you may have to be patient until it comes to your region.