I've occasionally thought that it might serve a purpose to add a layer of security by obscuring the back-end interpreter through a remapping of .php or whatever to .aspx or whatever so that any potential hacker would be sent down the wrong path, at least for a while. I never bothered to try it and I don't do a lot of webserver work any more so I'm unlikely to.
However, I'd be interested in the perspective of an experienced server admin on that notion.