A Broadway show sells tickets for weeks before it officially opens, and critics are asked to wait. The preview period exists because a musical cannot be finished in a rehearsal room.
Rehearsal cannot simulate an audience
Companies rehearse for weeks in studios, but timing built there assumes silence. An audience laughs, applauds and gasps, and every one of those reactions consumes seconds.
Comic material that landed in rehearsal can die in the house, and material nobody expected to work can stop the show. Only performance reveals which is which.
Previews therefore function as controlled experiments in front of paying audiences, with the creative team taking notes from the back of the theater every night.
The technical transfer happens late
Sets, automation, lighting and sound are assembled in the theater only shortly before previews begin, because the venue is usually occupied until then.
Technical rehearsals then work through the show in fragments, and the first full runs with all elements together often happen with an audience present.
Problems that surface at that point, such as a scene change that runs long or a sightline that fails from the balcony, require physical rebuilding rather than a note.
Changes are made between performances
During previews a company may rehearse in the afternoon and perform a revised version the same evening, absorbing new lines, cues and staging within hours.
Songs can be cut, added or moved, and scenes reordered. Performers work with the understanding that the show they play tonight may differ from last night's.
This is why preview audiences are watching a genuinely different product from opening night, and why productions state that the show is still in progress.
The freeze establishes what critics review
Some days before opening, the production freezes: no further changes are made, and the company plays a fixed version to settle into it.
The freeze exists so that opening night reflects a stable show, and so that reviews describe something audiences will actually see afterward.
Critics are invited from the freeze onward, and the convention that reviews publish after opening protects a production's ability to keep working during previews.
Economics make previews necessary rather than optional
Theater rent, salaries and running costs begin the moment a company moves in, so a long silent development period is unaffordable at Broadway scale.
Selling preview tickets recovers part of that cost while the work continues, which is the trade the discounted preview price represents.
Preview length itself is a budget decision, since each additional week buys more refinement and costs another week of running expenses before any review exists.