Virtual domains can have more than one use. If you want them to add "stickiness" to your site, give them to your visitors in exchange for services. Need someone to sponsor your "Win a FREE t-shirt" contest? Trade free web hosting for a few t-shirts ... If you're a web designer, include free hosting in your web design package ...
Virtual domains can also be used to separate your site content. Let's say you have a lot of visitors who only use your forums and you want to give them an easy address to get there. Use forums.yourdomain.com instead of yourdomain.com/UltraBoard/UltraBoard.pl.
Maybe you're looking to make a few bucks - this one's too easy. ;-)
Note: We set aside 10 megs of space on your portal for each virtual domain. If you have a Portal Pro, which comes with 200 megs of space, you can have up to 19 virtual domains (that's 10 megs for your portal and 10 megs for each virtual domain). Also, you must have your own top-level domain name (not something.searchking.com).