Dressing up for the Theatre
I first noticed this last year when I got to college. When there is a school sponsored trip to a local play, a lot of students like to dress up. I find this rather confusing. We just go to little theatres in nearby towns and stuff, nothing big-city. And it's not like it makes a difference what you wear, because we're just audience members.
I asked my mom, who is NT, a while ago, and she said a lot of people just consider it an excuse to dress up because they like dressing up. I personally find it a waste of time and energy to dress up and look nice just for some local play. Why does everyone else bother to?
I like going to plays, but I don't go with my school anymore because I feel underdressed and that everyone else is being illogical by dressing up.
There are two key reasons that I know of.
First, for many people, the theatre is a bit of an event to go to: they don't go all that often, so it is a considered an excuse to bring out the "quality" clothing.
The second key reason is that it is considered a sign of respect to the stage and the performers. Live theatre has always had a bit of a (false or not) reputation of "respectability," and the unofficial dress code has tended to be part of this. To be honest, it tends to be rooted more in class issues that stretch back hundreds of years, but it is still ongoing.
I am not sure if this is true, but the rumour used to go around that Patrick Stewart, while performing on stage, stopped to yell at two people in the front row dressed in Trek gear. If true, it was completely rude of Stewart, but does illustrate how seriously some take the dress code.