Well not offhand, comments aren't supported so workarounds are used instead. But I would find it very convenient if I could leave comments on dependencies and scripts. Or even various engine requirements.

Sure you can write everything in another file or put the comments in the commit message. But out-of-band comments are more easily missed or lost. If the package.json got rewritten by `npm install` you'd lose the comments. Inconvenient, but that's trivial to fix at commit time.