The idea is that you still conform to the convention of the project (Rails, Spring, etc.), not the IDE, and the FOSS CLI tools etc. can still understand the pro

The idea is that you still conform to the convention of the project (Rails, Spring, etc.), not the IDE, and the FOSS CLI tools etc. can still understand the project, but so does the IDE (because it is made to understand the convention, e.g. how the annotations/XML files work, how to get a list of all your Rake tasks). By not sharing IDE project files, not having any IDE-specific machinery in the project, but still expecting CI to be able to run it, being able to theoretically hack on the project and get results (build, test) even with just bash and grep and nano, you get the best of both worlds. Modern IDEs are better at this than they ever used to be. I was really anti-IDE for a long time, but JetBrains' products made it make sense finally.