I started making a post on another post about how high to make a second workbench. I have three or four workbench/surfaces and plans for at least three more, partly because I work in multiple locations still... I think a discussion of the various bench/work support needs, might be of use. I have no claim to expertise but I have gleaned some interesting thoughts from reading threads on this topic over the years. Maybe others would care to mention their specific needs or thoughts?
First, isn't the point of the knuckle high rule that the height, torso, arm length of different wood workers vary, making all this information dependent on each individuals "build"? Some people use the knuckle of the pinky which is a little higher...
There are two or three types of work that I most frequently find mentioned as benefitting from differing bench/work surface heights. Planing certainly works easier on a steady bench of appropriate height. Typically benches built for planing may benefit from a lower surface allowing the worker to reach arms over the top of the plane. A relevant consideration for a planing bench is the type plane that will be used. Some wooden planes put their users hands significantly higher than most metal planes. How and where the plane is gripped may also change the level the workers hands will work at and bare consideration. Festool tools like the skill saw, jigsaw and router may require one to "hunker" over the tools. My Festool MFT (Multi-Function Table) is 32" and my current "other" bench is 34". Yea, yea Festools may not be "hand" tools, but they are more "handy" than the big stationary tools and arguably as versatile, particularly with the "dustless" vacuum system, so I mentioned them. I do significant amounts of construction work and the Festool tools are part of my effort to eliminate a tablesaw and a chop saw in favor of more "handy" tools.
Some fine chisel and fine saw work may benefit from a higher bench top. Getting work closer to the eyes and more even with bent arms at the lower torso may be an advantage. This is where some woodworkers may elect to use a Moxon Vise on a "standard" height bench. Although some people may deal with some of these issues by working from a seated position. more people seem to prefer getting their entire body involved.
A third set of issues may benefit from a lower work surface. Cabinets and furniture are often large objects who's tops may be "out of reach" if placed on a standard or even short planing bench. Cabinet/Furniture makers may use benches or sturdy tables at much lower heights. Those who do significant amounts of sawing with standard length hand saws may also build benches at much lower heights which may allow them to secure pieces with their legs and arms.
One other consideration is working with sheet goods and larger lumber. Unless we have a very large, wide bench these materials will need some sort of support system. They can be placed on sacrificial 2x4 lumber on the floor. Working at that height was not comfortable even when I was a limber youngster. Some other bench/out feed table may be required to deal with these larger pieces of wood regardless of whether or not one works them with hand or power tools. I have several out feed stands, none of which is sturdy enough for me to be comfortable with.
There are adjustable height benches. I have a Norden Adjust a Bench with locking casters that I am torn as to how to put to work. I originally bought it with the idea of making a lighter bench top with plywood and MDF or hardboard for cutting sheet goods and providing out feed support. I was thinking about various sacrificial surface options that might be swapped out for different work. Then I found threads here in which people have made heavy duty more conventional benches with this system. I have also made plans to make a more conventional bench so I am stuck trying to figure out whether or not to build two work surfaces or one " Swissknife" bench....
Rather than getting caught up in another workbench thread I thought a discussion of what work people find benches/worksurfaces helpful for and why might be more to the point.