At my Crealde Urban Sketching class, I had the students sketch the darkened studio next to our classroom.
Black garbage bags and clear plastic were taped together to cover an open doorway to the studio, creating what looked to me like a scene from a horror movie. A small portable AC unit was set up to cool the room.
Part of the reason I had students work in the darkened room was that many student water colors never get dark enough. When water color dries the wash gets a bit lighter. With experience you adjust for this and paint a bit darker.
If you have a pure black on the page next to the pure white of the paper, that become an eye magnet. It is hard not to look at the high contrast. My goal is to get my students to create a full range of values from pure white to pitch black.
I have no idea what this doorway repair work was all about, but it offered a great way to show students how to look for a definite and clear light source in a scene. I usually make a concerted effort to be sure a person is in each sketch, but in this case it was liberating to just set the scene.