Why do you think so many startups have a private beta? You don't know whether you get in and that makes you want to try it out. Glitch also does this fantastic

Why do you think so many startups have a private beta? You don't know whether you get in and that makes you want to try it out. Glitch also does this fantastically well with their playtest weekends. They don't tell you in advance when they are, so if you want to participate you need keep an eye on them via twitter or the site itself at all time.