It lets you install and restoreclient-side packages, including JavaScript and CSS libraries.
there's already a rich ecosystem for this kind of thing. NuGet is great for server side libraries (and some client-side) but there are so many more CSS and JS libs on npm and bower
Each time a new version of jQuery or Bootstrap comes out, though, someone must create and release a NuGet package for it. But because Bower can use semantic versioning, as soon as a tool is released and tagged on GitHub, Bower can use it; no need to wait for someone else to package it up in a NuGet package.