HOW TO: Get Traffic to Your Website Part 5 - Microsites and Subsites
Having multiple subsites and microsites that point back to your main site can be good and bad. On the good side, you can customize these sites to target audiences better. You also have inbound links coming from your own sites coming back to your main web address. The bad news is that unless you are careful, it is easy to confuse your customers as to what site they are on.
Often subsites will click through to a main site and people get lost. You also are diluting the power of your main brand and website when you have subsites. It is better to have all of your traffic going to your main domain so it gets credit for being a popular site with search engines.
Sometimes it makes sense to have a landing page for a campaign or promotion. These can be very effective when they have a single purpose and a clear focus. Make sure that you explain what is happening and where people will go when they click if you send them away from the page.