Visual Studio already had the ability to recognize and quickly navigate (with Ctrl LeftClick) to URLs embedded in the comments of the code. This is great, but what is really needed is a way of referring quickly and easily from one part of the code to another part of the code (or to architectural documents checked in with the code). That is exactly what the HyperAddin does: It makes creating and following hyper-links in your code trivial. Now you only have to explain something once in a comment and refer to it many times (got lots of overloaded methods? They should all refer to the 'master' that does the real work).
Hyperlinks in your source code are one of those features that once you have used it, you wonder how you ever got by without it. If you use Visual Studio, you really should try it. Anyone reading your source code (including yourself!) will thank you for it.