How was the camera obscura used to help artists?

The camera obscura was a very early form of a device now called an "opaque projector". By looking at an image of a 3-dimensional object projected onto a sheet of paper, an artist could more closely reproduce its details.

The use of a camera obscura is considered by some artists to be a form of "tracing" so its use as an artistic aid wasn't always well-publicized.

*I would offer that the reason had more to do with reproducing perspective as accurately as possible when representing 3 dimensional scenes on to 2 dimensional art. At one time in the artist's world, perspective was of utmost importance because there was a sector of artists and collectors that concerned themselves (perhaps too much) with spatial relationships. Two renderings of the same scene could be interpreted by the artists in different ways. So how to know which was "right". They thought the answer lay in the use of the camera obscura, which enabled the artist to render the scene on paper, thus capturing the "real" perspective.

Micron