When should you let your audience “in” on what’s happening? When should you keep it a big, heavy, elephant-in-the-room secret? All plays have some elements of both – one of the jobs of the playwright is to measure the balance between what the audience knows and doesn’t know, and how and when to withhold or impart information. Sometimes when you want the audience to know something, the quick and dirty way to accomplish that is to just tell them. Why bother with fancy tricks to get your most important exposition across? Want your audience to know that one character thinks the other one is crazy? Just tell them!
[Aside] Though this be madness, yet there is method in ‘t.
This is a nice little essay about the use of direct address in a few contemporary plays.
A couple of my favorite examples of plays that use monologues spoken straight to the audience (aka an aside or in a slightly different sense, a soliloquy) are Thornton Wilder’s Our Town and John Guare’s Six Degrees of Separation. These plays are a story being told and acted out for the audience. The play is aware of itself being a play, and the characters consistently break the fourth wall. Like a Greek chorus, they narrate story as it goes along. So, yes it is an old contrivance, and a very effective, if blunt, instrument. And yes, sometimes blunt is an excellent thing.