It's up to the optical designer to ask the right questions in order to know the needed parameters the client does not know.
Parameters are nice. They make a goal that is much easier to reach than an invisible one. Optical designer knows what parameters would be nice to know, but asking them from client who might be the idea man or organizer, might not know what they might be. Then it's up to the optical designer to ask the right questions in order to know the needed parameters the client does not know.
For imaging optical layout, resolution is one of the most important parameters. This is also very easy to determine - find out the sensor resolution (or decoder disk hole size) and top that. Diffraction limited, smaller than the pixel size or MTF of over X% at sensor's Nyquist frequency, whatever works. Of course if cost is also a matter to consider (which it always is), one could find out the best resolution customer has had in the past and top that one instead as a minimum goal. One needs known minimum goals reached before advancing to the next goals.
Field of view and sensor (eye) size helps to limit blurring from coma, astigmatism and field curvature. If one has a visual application, it doesn't matter all that what the blur is at 22mm image height, or when the blur is still smaller than the pixel size. Of the field aberrations, distortion is also noteworthy - is the end user going to use the optics for i.e. mosaic imaging? Machine vision doesn't care about distortion, as it doesn't degrade image quality, and line/area scanners etc. can be calibrated to a distorted image.
Consumer optics are most demanding, as clients want the best and are more readily to complain about failured products. In some cases, though, the good reputation has come from simple tolerant optical layout that gave feeling of reliability and ruggedness - more headaches for the optical designer. Low tolerances are easy enough to design, but the number of usable glasses are fewer. With consumers, optical glass needs to be resilient as well, since optical devices (with no electrical or moving parts inside) are mostly assosiated with long product life cycles. In these cases, acceptable resolution tends to be adequate more than high-performer.
Basically, the client should but doesn't always know the most important parameters that he needs, or at least rough idea of what they should be. Problem for the designer is that all of them need to be controlled, and therefore known. And knowing exactly what is going to be on the image plane helps with this.