Most people outside the truly intimate theater goers and professionals don't know what a preview is. The definition itself is somewhat vague. Basically they are ticketed performances (usually cheaper than the run) before opening when the press comes. They are something more than an invited dress rehearsal - which is just that, a rehearsal - and a full performance. Some production elements and etc. can still be in the process of solidifying, but for the most part, and audience member is seeing the actual show.
Which always beg the question, why isn't it a real show? What's the point of previews?
Many companies in this town don't have them. Due to their rental contract or simply the amount of money the have to spend on any one production. And while they will have one or two previews just to keep up with tradition, those previews really don't make much of a difference.
What previews are helpful for is getting the kinks worked out with the audience. It can sometimes take a while to get acclimated to laughter and applause (or lack there of). And if the first time the actors are facing this is opening night, then the addition of the opening night nerves along with facing a big unknown can keep the show from living up to its full potential. You may ask "well wouldn't this just happen at the first preview anyhow?" The answer is yes - but the bigger point is that the Jeff Committee and a slate of reviewers definitely won't be sitting in the audience at a first preview.
And in this town that is so over saturated, one bad review or the lack of a Jeff Recommendation can seriously affect how well the box office does for a show. And there are a thousand reasons why these people have to see the show as soon as possible. The sooner the good reviews are in the sooner the show will have people wanting to pay for tickets. The Jeff Committee has to come opening night to all time for the rest of the judges to see the show should it become reccomended during the short 18 performance runs. There are just so many shows going on that it's not possible for all publications to get reviews out right away.
But in the end, are they really seeing the show that is going to blossom by the second weekend of the run? How does the investment in time for previews pay off in the long run?