Actually I have to kind of disagree with this EelKat. Yes you are right that people need to change sets and costumes. But that isn't the only reason WHY there exists such structures. And even though the construct exists, in part, to aid the technical aspect of the show, it is important for the writer to keep that construct in mind with writing the story arc.
For example in the big broadway musicals, there is a convention to end Act I on a cliffhanger and with a huge musical number. I guess so people want to come back and see Act II and don't decide to go home instead. But you can't just suddenly have a cliffhanger, you can't just "put in big musical number here". There has to be a story leading up to that point.
You are also working a little backwards. A playwright doesn't write in an Act break because someone needs to change the set. Someone needs to change a set because there is a break in the acts. As in, let's say, the playwright feels there ought to be three different setting in their play, well they think that not because "Gee I guess we have to give the stage management something to do, better give them a scene change" but because a scene change would best serve the story. (also I think you are confusing "Act" with "scene". An Act can have many scenes within it, many set changes and costume changes. An Act is more akin to a chapter in a novel. In a chapter you can still have many different scenes, often broken up by stars or a blank line in the text).
Also it is important to note that the five act structure? That came from a time where there was very little scenery to change and no intermission at all. When Shakespeare was performed, there might be a chair or something brought on once and a while (and they never stopped the play to do this, but merely did it under some other actors performing. They performed in broad daylight, so there were no blackouts or anything to let someone sneak stuff onto stage), but they were basically on a bare stage. And if anyone had to go the washroom they would either leave, or do it standing where they were (at least the groundlings would). So it makes absolutely no sense to say that the five act structure was created so that the techies could change scenes! The five act structure was a literary device, an old fashioned version of our now much more modern structure: rising action, climax, denouement. We wouldn't say that this structure was there so that some techie could take care of something. No it's there because the result is a pleasing story. So too was it with the five act structure.
Nonetheless you are correct in that a playwright must keep such things as set changes, intermission, etc in mind while writing their play. In fact I find it very interesting reading plays by people who have never written one before, or aren't that familiar with theatre. In movies you can write about whatever you want (especially now with how awesome special effects are). But plays still have to take into consideration so many other things than just story:
What kind of stage is this play going to performed on?
What kind of "effects" do you need, and how could they conceivably be done?
Will you have an intermission at all, and how will you put it into the show so it doesn't disrupt the action?
And many many more questions.
In the end though, they all still must be resolved by writing an awesome story. And it's the story arc of the play, divided into however many acts you wish, that will resolve all these issues.
