It's purely for aesthetic purposes - I have an ASP page that runs on each user's PC when they log in each morning that adds their PC details to an asset register.
I wanted a way of guaranteeing that the messages telling the user what was going on were in the middle of the screen so they couldn't miss it and not close it down.
Having done a bit more research though I see there are a lot of people out there that aren't keen on the idea!
In the end I used:
<script Language="javascript">self.moveTo(0,0);self.resizeTo(screen.availWidth,screen.availHeight);</script>
cheers
Ed