It doesn't, the 2x6 "fad" went on long enough that you can buy doors and windows sized to that standard. You can use any thickness of wall you want, but if it isn't same thickness as a 2x4 or 2x6 you will have to put forth extra effort to make your doors and windows fit. Use a standard dimension wall and you can buy those to fit.
Not a big deal for a qualified carpenter, but enough extra labor that you will pay extra for it in addition to materials.
This is clearly regional. I'm in the US where we don't measure in MM. Different parts of the world will have different standard sizes. you might be able to import a US standard door if you wanted to use 88.9mm widths for some reason (there is more than enough tolerance to round that to 90mm), but it would be extra effort on your part. The hard part about importing it often codes are designed to limit this, so even if the US door is better it might not meet some standard that shouldn't even required. (this goes both ways, US standards often require things just to limit imports as well)
Not a big deal for a qualified carpenter, but enough extra labor that you will pay extra for it in addition to materials.