More to the point - when to go outside the normal boundaries of the manufacturer's lens line. When to break away and decide to explore new territory. When to take command yourself.
Well the manufacturers of digital camera bodies all give you lenses from their stables that are perfectly suited...each to its own mount. They have generally worked long and hard to have a wide variety of optics in all sizes and focal lengths - and they will generally have a range of prices for them as well. You pays more, you gets more, but since the lenses are good to start with, the extra costs can sometimes be a diminishing return.
Enter the independent lens makers - companies who may have a license to use the mount of a certain camera but are not restricted as to what the other specifications need to be. They can search for the niche market that wants a particular focal length or maximum aperture. They sometimes elect to leave out auto-focus and auto meter connections to make the lens universal - they figure that if you are advanced enough to want a specifically special lens, you are capable of focusing and stopping down yourself.
The stop-down question is not even addressed in many cases - you get a good old click-stop aperture ring and are in command of it every time you shoot. If you are a maximum-aperture freak you just cello-tape the aperture ring open and let it go at that...put the camera on aperture priority and let it sort out he exposure. In most cases it will guess better than you can anyway.
So here's what I found in the special shelf - a Zhong Yi Creator 85mm f:2 lens. Totally manual, but with the sort of feel and finish that make it as good as the big boys. I think it would be the lens for someone who wanted to do street portraiture quietly and from a distance. So, of course I used it for lots of other things...
Out on the street it does react quickly - no AF or metering to get in the way, and you can stand and stop cars as they go past at 50 kph. Choose a high enough ISO and you can stand at a race course and stop the action good. f:2 lets a lot of light in.
Likewise if you use it as a stand-off standard lens. It would be a an excellent optic for separating the background from a face - hence the street portraiture.
And lastly, it is actually quite nice in a studio set, though here it is stopped down to f:11.
The action of the lens is extremely smooth and the aperture ring has clean click stops. Oddly enough they have an f:16 stop between f:11 and f:22 but do not mark it as such.
Note that there is a good bayonet fitting for a lens hood but none is packed in the box - presumably it is an extra.