It's quite common practice for webmasters to do that sort of thing. Often, it's simply a redirect to a specific version of a site (say, if Flash or Javascript is not detected, it would put you to a HTML and Images only version). They sometimes also use this to show referrer information etc.
Another thing webmasters sometimes do is buy multiple domains. In this case, I think Google are hosting this on gmail.google.com but, quite rightly, spotted that it wasn't exactly the snappiest URL. So, they bought gmail.com and simply pointed it at the original server.
The latter part of the URL to me looks like a bit of validation or verificaction of some sort. An additional security measure, perhaps?