As search innovation rolls out to the user, the beta release has been a tried and true way of testing the waters. Currently, there are dozens of different flavors of search in beta, including a significant portion of Google usability. Beta releases were originally a quality assurance exercise, allowing real users to identify bugs in a new product.
Today, the advantage of beta in search is that it's a relatively low risk way to test the appeal of new search functionality and interfaces with real users. Beta release is to technology as a test market is to advertising. A beta interface can be thrown up without impacting the main site, which continues to produce the bread and butter revenue. The hope is, of course, that word of the beta will spread virally through the internet and the developer will find their beta release turns into the next big thing online. You pretest with users, find you have a home run, and when the time is right, you throw the switch, incorporating some or all of the new technology into your mainstream product.