I.e. that's why we now have NPM and Bower in JS world:

At Backend:

• You want to have hierarchical package structure
• You might have globally installed cli versions, but you might want to use locally installed packages instead (e.g.glob &grunt) to ensure certain versions.

At frontend:

• You don't want to ship multiple versions of a same library, apart from rare exceptions (e.g. Angular v1 and Angular v2 in the same app).
• You want flat package structure
• Because of above, versions might need to be negotiated loosely

More?