There are legal ("asset protection") reasons that many taxi companies have each car as its own LLC, or ski slopes register each slope as its own LLC. So if someone has a legal problem with one, and you can really show that you've been treating them separately in your bookkeeping and bank accounts, it's clear to the creditor that they're not going to be able to go after everything you've got.
Then of course as they grow there are reasons why you may want to sell one of your websites, but not the others, etc.
The reasons why to do it are vague and different for everyone, but to me, it's a "can't hurt" and "why not?" type decision.
You go to LegalZoom (or any similar service) and pay $400. They mail you a certificate you bring to your bank to open a separate account. Total spent: $400 and 30 minutes of your time.
California is an exception: They make you pay $800 per year to the state for each entity. Most states it's only $50 and some states (like New Mexico) you never need to renew or pay any annual fees at all. If you're a virtual company with no physical office, you can set up your LLC in any business-friendly state you want. (Delaware, Nevada, Wyoming, etc.)
Outside the U.S., British Virgin Islands is the quickest, cheapest, easiest place to do it: http://www.google.com/search?q=bvi+corporation