Right, but modern text editors and IDEs automate the lookup, and failing that, you have the type right there so you can quickly and easily figure out which file

Right, but modern text editors and IDEs automate the lookup, and failing that, you have the type right there so you can quickly and easily figure out which file contains its definition. I use Python every day, and the sad (but improving!) state of typing is one of my biggest grievances. Documentation doesn't close the gap because it is often incorrect or outdated; I've added more than a couple production bugs because I trusted the documentation. A type checker is necessary to keep the type annotations in line with the source code.