node.js - Why use peer dependencies in npm for plugins?

Why does, for example, a Grunt plugin define its dependency on grunt as "peer dependencies"?

Why can't the plugin just have Grunt as its own dependency in grunt-plug/node_modules?

Peer dependencies are described here: https://nodejs.org/en/blog/npm/peer-dependencies/

But I don't really get it.

Example

I'm working with AppGyver Steroids at the moment which uses Grunt tasks to build my source files into a /dist/ folder to be served on a local device. I'm quite new at npm and grunt so I want to fully comprehend what is going on.

So far I get this:

[rootfolder]/package.json tells npm it depends on the grunt-steroids npm package for development:

  "devDependencies": {
    "grunt-steroids": "0.x"
  },

Okay. Running npm install in [rootfolder] detects the dependency and installs grunt-steroids in [rootfolder]/node_modules/grunt-steroids.

Npm then reads [rootfolder]/node_modules/grunt-steroids/package.json so it can install grunt-steroids own dependencies.:

"devDependencies": {
    "grunt-contrib-nodeunit": "0.3.0",
    "grunt": "0.4.4"
  },
"dependencies": {

    "wrench": "1.5.4",
    "chalk": "0.3.0",
    "xml2js": "0.4.1",
    "lodash": "2.4.1"
  },
"peerDependencies": {
    "grunt": "0.4.4",
    "grunt-contrib-copy": "0.5.0",
    "grunt-contrib-clean": "0.5.0",
    "grunt-contrib-concat": "0.4.0",

    "grunt-contrib-coffee": "0.10.1",
    "grunt-contrib-sass": "0.7.3",
    "grunt-extend-config": "0.9.2"
  },

The "dependencies" packages are installed into [rootfolder]/node_modules/grunt-steroids/node_modules which is logical for me.

The "devDependencies" aren't installed, which I'm sure is controlled by npm detecting I'm just trying to use grunt-steroids, and not develop on it.

But then we have the "peerDependencies".

These are installed in [rootfolder]/node_modules, and I don't understand why there and not in [rootfolder]/node_modules/grunt-steroids/node_modules so that conflicts with other grunt plugins (or whatever) are avoided?