There are a few reasons for pseudenyms I think:
1) You can create a genre/sound in the clubscene of your own. Ie peeps think its bigger than it it is with all these 'other' poeple making a similar sound.
2) People like to test actual reactions to thier tunes every now and again without the fear of people 'just' buying it for the sake it saying 'Pianoman' on it or whatever.
3) You can produce more varied genres of house under different names and therefore not be boxed into a particular scene. Scot Brown had million of other names cos he made house, funky house, happy hardcore, techno etc and not just hardcore on its own.
4) You can create deals with record companies. They often like you to sign a contract that ties you down to not recording material under that name for anyone else. So, if you set up as someboy else you can probably get away with still earning the spondulix remixing other records whilst raking it in from your existing name.
5) If a tune bombs, nobody needs to know its you! lol.
6) Its adds an air of mystery around the scene, who might be what, who's remix is really by who? etc etc which kinda drags/sucks people into finding out more about the stuff - like this very thread.
Im sure theres others, Im not even in the music making circle but I bet at least half of those above apply at some point.
Or I could just be sprouting shite - one or the other! hahah
See ya
Sirius.