Sketchup is a simple 3D modeling tool, useful to put quick ideas on "paper" in a digital platform so that they can be easily shared and understood by others. Sketchup is also very useful for 3D printing for casual users, with a large library of 3D objects available for printing.
Their online app is a simplified version of the true Sketchup software, and is opensource and available to all. This is a huge benefit - the app has much of the same functionality as the full software, and has the ability to save your projects to the cloud. This has proved very useful for smaller personal projects such as home renovations.
Also of note are the good walk through/fly through capabilities that allow you to see your projects from realistic perspectives. Review collected by and hosted on G2.com.