By "overture," I assume you mean the portions of text in the beginning of Arthur Miller's play The Crucible in which he gives readers some background information on the characters and on Salem. While these explanations are a distraction from the opening action of the play, they are also necessary to put the events which follow into their proper context.
Here we learn about the prevailing issues in Salem: greed, paranoia, and fear are the backdrop for the hysteria which is to come. The descriptions of each character help establish their motivations and prepare readers for the role each of them will play as the story unfolds. To that extent, the "overture" is invaluable for readers of the play.
One thought I always have when reading this play is how much more difficult it is to perform this play than to read it. The actors have to work without the audience having the benefit of all this valuable information and insight. For example, the person who plays Thomas Putnam must somehow reflect decades of resentment and anger; and the person who portrays Rebecca Nurse must portray a woman of such godliness that Reverend Hale--from another town--has heard of her as an example to be followed. For readers, though, the "overture" is an invaluable resource in understanding the context of the action and the motivations of the characters.