[quarterto]

Dependencies. For example, Backbone depends on Underscore, and simply writing <script src="underscore.js"/><script src="backbone.js"/> encodes nothing about that fact. There's nothing stopping you updating Backbone to say a 2.0 that requires features in Underscore 2.0, but forgetting to update Underscore. On the other hand, bower update backbone will see dependencies: {"underscore": "^2.0"} in bower.js and update Underscore as well.

[sphildreth]

Is it something like NuGet for .NET packages but for Javascript? Does that mean that an ASP.NET MVC application using NuGet wouldn't really have any "need" for Bower?

[atonse]

It's similar but for you to not "need" bower but get the same from NuGet, you'd have to make sure that every JS library you need is on NuGet.