This is done:
- to reduce the exposure (or compensate for changing to a slower shutter speed, keeping the exposure constant),
- to increase the depth-of-field, or,
- in an SLR camera, to switch from the aperture fully open (for viewing/focusing) to the taking aperture (for depth-of-field preview, "stop-down metering", or taking the photo). On early SLRs, the lens had to be manually stopped down; later cameras coupled the diaphragm to the shutter release to stop down automatically.
Stop-down metering is a technique used in TTL systems where light measurements are made through the lens, set at the taking aperture. This was used on all early SLR TTL systems; the alternative - metering with the lens fully open requires the metering system to "know" the taking aperture and compensate for it when determining the shutter speed. Full-aperture (or open-aperture) metering requires a coupling mechanism to communicate the taking aperture from the lens to the metering system (or, if set by the camera, to "tell" the lens which aperture to use); this coupling (and so full-aperture metering) was not generally available on SLRs until the 1970s. Many cameras with open-aperture metering still feature a stop-down metering mode - to allow for lenses or accessories without coupling.