A few years ago, a common but spammy practice was to buy as many domains as possible that contain your keywords. The idea was that you could then have many sites showing up in the search results since it was often easy to rank high for “favorite keyword” with an Exact Match Domain (EMD) like “favoritekeyword.com”. If that didn’t work out, you could have the extra domains redirect to your real site and still get some of the benefit. And by owning every possible keyword matched domain, you could avoid future competitors coming along and having a really easy time ranking for some good keywords. Google’s EMD Algorithm Update eliminated some of that and made it more difficult to succeed with this somewhat spammy strategy.

Regardless of the EMD update, Google’s increased focus on quality makes it necessary for all of those sites to be pretty good, or have some really effective link spamming in place in order to rank well. Even then, it isn’t likely that these sites would stay at the top of the search results for very long unless you put a significant amount of effort into making those additional sites useful and full of good content. It is much better to put your resources into improving your real website.

Google’s Matt Cutts made a video about this about two years ago and Google has advised against it other times, but people still think lots of doorway sites on several domains is a good idea.

Like most things in SEO, multiple sites can still be used in moderation (as in just a few of them) if the sites are different enough from each other. Maybe they sell different products, or are aimed at a different type of customer. As long as they are unique and good quality, you can still make use of them. Or maybe your company’s name happens to be a commonly searched competitive keyword, therefore it is actually a branded domain, too. But don’t expect multiple domains to count for much in terms of SEO when linking them together.