With a monocular, you look through the viewer with one eye. A monocular is namely a viewer with 1 eyepiece and lens. This means that the viewers are more compact and lighter than the roof and porro. The advantage of looking with one eye is that you can continue watching your environment with your other eye.
However, your perception of depth is disturbed by a monocular. Monoculars are very suitable for bird watching, observing wildlife and watching a stage show.