Angela,
If I understand your question, you're asking "why would anyone mask a domain and forward it to another domain?" Is this correct?
The way I use this technique is... well, honestly, after I got
www.milagroresearchinstitute.com up and running, i thought of a better name for it...
www.instantchange.com. shorter. easier to remember.
so i purchased the instant change domain at godaddy, and then had it forward to milagro research institute's home page. try it... you'll see that your IE address bar still reads "instantchange," although you're actually at the other site...
there are some smooth tricks you can play in affiliate marketing with this... you can embed your cookie and send your client to a main sales page, for example...
i'm sure some of the others on this forum can think of other uses, but that's what i know.