Official docs say it’s for
Packages that are only needed for local development and testing.
Umm, okay. Not 100% clear there. Some articles mention things like ESLint or Jest (k, I’m onboard there) but others mention Babel or WebPack. I get that you don’t need WebPack libraries to be loaded in the browser but how the hell do you bundle up your code without it? When you use npm ci or npm install you’ll get all dependencies but isn’t it good practice (in a CICD environment) to use --omit=dev or --only=prod?
You only need to worry about devDependencies vs dependencies if you are going to publish the project you are working on as an npm package. If you are making a webapp or something else that you will run, then it doesn’t matter.
Not always. If you’re publishing your app as a docker image, you want the final image to exclude dev depending to be a small as possible.