It's likely for the print media, like handbills and TV adverts. When we were studying costume design, one of the things we covered was how zippers were a boon in theater costuming. But, when movies came out, the zippers could be seen.

There's a famous, rather close up shot of Olivia de Havilland, in a dress with buttons from the neck to the hips. She was ascending a staircase and the zipper was obvious.

You just can't fake it close up, nor in stills. And, if the nearest customer seats are too close, sometimes the audience is practically on the stage. Some small venues or venues are close. Others, the orchestra is to the side instead of between the audience seating and the stage, so customers may be closer. In our school theater the orchestra is in front of the stage; and, there's a wide walkway between the pit and the audience seating.

Our teacher made us learn period specific techniques. He said the Orpheum (where the students actually in costume design as a career choice usually intern), they required full period compliance. The audience wasn't any closer; in fact, they were probably further from the stage than our school theater. However, it was common to see audience members with their little binoculars. You know the type – they're the ones who see everything. And, they tend to be the persnickety folk who must point out every flaw as they see them.