Sometimes when decorating a home we mix ideas from various styles as we desire our own taylored ambient.
A classic chair may be a good match with a minimalist interior or in a Neoclassic palace we think of some Urquiola furniture and Portuguese ceramics, and so on. What about what I call the finish line accessories like the the curtains?

The rule that follows aesthetic and functional paths says to always place them in the bedroom and optionally in the living room. It obviously  depends on lots of factors like the home architecture or the interior design project, but in the bedroom they are a must.
Curtains can be used in bathrooms as well, as long as they remain simple and plain.
As for the living-room, it depends on the home project and its location. Winner of the Inside best interior design prize, the apartment by David Kohn in the heart of Barcelona doesn’t have curtains (see photo below), maybe to better display the beautiful Gaudi supplier ceramics or the whereabouts of Ciutat Vella. I must add that in this case the warmth of some curtains is missing. 
Below there’s an excellent mix of  XIX century French classic architecture with XXI century  design; the curtains are subtle and classy, leaving the scene to the rest of the space (4th photo). 
The curtains can be also used to camouflage oddy window frames or sometimes to give intimacy by hiding a door or entrance. They can divide two spaces in a smoother way respect to the doors.
While the outdoor curtains can dress-to-impress even a small front terrace like the one from the below photo.Immagine2-1024x579