Google is serious about virtual reality, a point which they have made very clear over the last 12 months since they introduced Google Cardboard at the 2014 IO developer conference. Since then, they have worked towards developing VR apps for use with the headset, and have even started an initiative called Works with Google Cardboard which is a set of guidelines for developers and designers of the platform, and to make it easier for consumers to recognize third-party VR headsets which are compatible with Cardboard apps. Google even revamped the Cardboard unit itself this year with a slightly new design which now fits phones with screens up to 6-inches as well as carries a new input button on the side. They also want to help developers learn the basics of designing for VR which is why they're introducing a new application called Cardboard Design Lab.
For developers interested in working with and design for the Google Cardboard or VR space, Cardboard Design Lab is a useful tool to become familiar with. It walks designers and developers through a series of VR design principles including "using the reticle," "spatial audio," "guiding with light," keeping the user grounded," and more, to help users with tips on creating unique and engaging VR experiences. What's more, is that Google has turned this learning tool into an interactive experience itself to be used with Cardboard as a VR journey through the different guidelines and tips which is, perhaps the most appropriate way they could have done so.
As the app was designed with Google Cardboard in mind, the actual Cardboard viewer will work with it, but Google states that it is also compatible with any viewer which works off of the Google Cardboard platform, so users won't need the Cardboard viewer specifically, although if they already have one they won't have to do anything else other than download and install the app onto their phones to dive right in. If you're a developer interested in designing VR apps, Carboard Design Lab should be on your list of must have installs, it's already available in the Play Store as well so you can begin work as soon as possible.