Survey of saddle roof knee wall height

But as the client of a gable roof house, I pay more if I build, for example, 20 sqm more floor area (top and bottom) than if I significantly raise my knee wall, right?

The builder makes more profit if you pay more for the same effort or if he has less effort for the same price. You will pay more if you get more living space (according to DIN or the Wohnflächenverordnung — depending on how the builder measures and bills this "extra") — so you will pay more for more square meters, no matter how they come about. What is called "knee wall height increase" in the additional price list is basically "calculated additional area due to knee wall height increase." A "knee wall height increase" in the sense of "more rows of bricks under the roof slope, but with the same type of roof truss construction" increases the effort and the price. If you build a town villa, i.e., make the attic a full floor by a "knee wall 260" or "knee wall 100%," naturally more rows of bricks are built and more (also calculated) living area square meters are created. However, above the just-built walls there is no elaborate rafter roof truss, but a considerably simpler "lid" as a tied roof, which is cheaper to build. In a cross-comparison between country house vs. town villa, you regularly get the "town villa" cheaper at the same nominal size. In a comparison within the respective category of "more height 'on top'" vs. "more area 'on top'," you will get "more height" more cheaply because you can get it "on top" "individually," whereas more area is also built and billed "below" at the same time. All variants have in common that with "free planning" the nominal square meters are the calculation basis because they are the easiest billing measure for the contractor to handle and communicate. With the "replanning of a catalog design" it is different: here, more width is more expensive than more length, and a knee wall height increase with a roof pitch change is always more expensive than without changing the pitch (even if the attic space does not become "larger"). Simply put, every change is more expensive the more dimensions it affects. The worst case is "length and width and pitch." For the staircase, the construction, formwork, and crane handling are relevant price factors. Here, too, the first and second laws of gastronomy always apply :)

So far, only windows and optical/furniture-related reasons have been explained here - I wonder if there are also technical reasons for a specific roof pitch (maintenance, rain, snow load)?
For example, with a standard house, we could choose between a 0.9m knee wall + 40º roof pitch versus a 1.25m knee wall + 35º roof pitch. Yes, no huge differences, but 1.25m does sound somewhat more practical, 35º is also supposed to be somewhat better in terms of photovoltaics, but do you potentially get significant disadvantages regarding tightness/maintenance and snow load, or are these magnitudes irrelevant – or should one really only consider space and appearance/windows?

So far, only windows and optical/furniture-related reasons have been explained here – I wonder if there are also technical reasons for a certain roof pitch (maintenance, rain, snow load)?

No, just recently in post #37 I explained (also cost-) technical reasons! By the way, in alpine regions, the danger of roof avalanches is countered with low roof pitches. In lowlands, pitches of 35 or 40° DN will not significantly influence how long the snowflakes remain.

but does one possibly encounter clear disadvantages regarding tightness/maintenance and snow load,

The limit for a smaller selection of roofing materials lies at 21/22° DN, so far from 35/40°.

For example, in a standard house, we would have the choice between 0.9m knee wall + 40º roof pitch versus 1.25m knee wall + 35º roof pitch.

90/40 also reaches 125 cm inwards after about 42 cm, 200 cm is reached after 131 cm; with 125/35 you would have reached that height after 107 cm. So only choose 90/40 if the storage room in the attic is crucial, and otherwise 125/35 for all other aspects. That is all I can advise you without knowing your project.