SketchUp is easy to learn and use, and is designed primarily for the conceptual phase of design.Why would Google want to offer such a product that doesn't have any obvious connection with search? I think it's about positioning. Partially because of the success of several existing products, such as Google Earth and Google Maps, and the growing importance of VR and 3D gaming as entertainment and social media, Google has realized that the 3D Internet will be a big deal. They want to have services and software in place to offer developers and consumers when the 3D Web really takes hold.
People use SketchUp to create architectural, mechanical, landscape, and woodworking models, as well as to design the levels in games, and the sets and storyboards for movies and television shows.
SketchUp can export models to a variety of 3D* and 2D formats. Many of the standard CAD programs can import and work with files that have been exported from SketchUp.
There's a pattern here. Google has developed some vital services for the 2D Web and text-based Internet that have become indispensable for hundreds of millions of people around the globe. SketchUp, and earlier purchases such as Keyhole fit this M.O. -- the company wants these services to become part of the suite of software tools that hundreds of millions of people use for the 3D Web.