When a photograph is 'taken' there are two adjustments for the camera lens to do,...ie; 'Focus' & 'provide an aperture'.....
Providing an aperture is done with the 'iris diaphragm',...an adjustable 'hole' thru which the light passes, modified by the glass components, to make a real (and hopefully 'focussed') image.
When the iris is set to a small (high number) 'aperture' it allows less light thru but the area of sharp focus (depth of field) is large......
When the iris is set large (low number) the light transmission is high but depth of field is minimal.
Thus,....a large aperture is preferable when trying to focus the lens as there is lots of light to see with and the shallow focus is easier to get 'sharp'.........
......This is fine, but to make a photograph one usually needs a smaller aperture to gain depth of field,...so the lens must be 'stopped down',..ie aperture closed, to actually take the shot.
If teh photographer is doing all this himself it makes for a very slow workflow so fast shooting of quickly moving subjects is usually out of the question,....and a more automated camera, which handles the stopping down, is needed. For many years these matters were a source of innovation among both photographers and camera makers who produced 'reflex cameras' which viewed and focussed thru the taking lens.
The first improvemnts (for miniature cameras) resulted in a spring loaded mechanism which 'trips' the lens aperture down slightly before the shutter 'takes' the shot........this has to be reset for the next shot.
The early Russian reflex cameras tested Russian ingenuity and they made a 'pre-activated' aperture ring which allows the photgrapher to shut down the iris to the decided aperture just before taking the shot. This is clumsy and not too quick but much better than a completely maunual iris. Even while the Russians were making this the world had moved on to 'fully auto iris' cameras which close and open the iris as part of pressign the shutter.
I don't know whether this also is a 'feature' of a pre-set aperture but I have found on my 44-2, the aperture can be 'pre-set' at a minimum value of say F/8.0 and then the iris can be manually varied anywhere from wide-open down to the pre-set minimum value. This is really good because it allows very fine of depth of field and / or exposure and it is really nice for video work too, allowing smooth transitions from blurred to sharp focus.