This product just blew my mind, since I know how difficult it is to keep up with Node dependencies. You could manually track updates of your dependencies and test whether things continue to work. But it takes a lot of effort. So most of the time, your software is in a Schrödinger state of being potentially broken, and you have no idea until you run
npm install and try the next time.
Greenkeeper handles the chores of dependency management. npm install and npm test is called immediately after an update. There is no doubt about the state of your software.