Why use invite codes?

You've built a cool website and want to get some real, actual, users. In this situation, some companies create a mechanism by which only people that have invite codes can log in, and then these people are given a few invite codes for their friends. What is the reason to use invite codes in this manner as opposed to just allowing anyone to sign up?

Is it a marketing ploy, attempts to limit server load, something to make TechCrunch write about you?

Thanks!