Theatre previews

Apart from being the performances before the 'official' premiere of a show and perhaps being a little unpolished, does anyone know why preview shows in theatres are normally cheaper than the official dates ?

I assume it is because there might be some slip-ups (although after long rehersals it should all be ironed out by then) and accordingly the price is slightly discounted.

Is it as simple as that, or is there some other reason for the discount ?

z

previews are to see how the show runs with an audience, for example there might be a big laugh from the audience at a particular part of the show which was not expected by the director or cast, if this happens the cast have to wait untill the laughter dies down to continue, the previews help the show or play find its proper rhythm if you what i mean