URL parameters or query strings are often used to carry information that can be used by hackers to do identity theft or other unpleasant things.
ASP.NET: HttpModule for Moving ViewState to Bottom
There are two reasons why it is desirable to do so.
Use an HttpModule to Spam-proof Your Website
Every time an email address is written on a website, it allows spam robots to collect it and abuse it. If you have a website (e.g. blog or forum) that displays the users e-mail address it would be a nice service to mask it for the spam robots.