>>1251084A taller headtube and or a taller fork with some spacers is stiffer than a tall quill
It doesn't really add adjustability (because not many people would cut a threaded fork), it just puts the baseline for height higher.
And in that bianchi's case, like many lower end bikes, it adds a handy point to put a reflector.
I think indurain's setup is quite interesting. For him, with a very large bike, it must have been a tradeoff between weight and stiffness, and he clearly favours a quite relaxed front end. If you saw his setup on some amatuers bike you'd probably call them a slow fred and laugh about it.
Here's a less extreme example. One spacer. That's conventional. I think there's probably some mechanical advantage to having one spacer, or it's just considered good form, like using a washer.
Some threaded headsets could not use spacers though. I have one like that with a very odd design, needlenose bearings and a pin spanner top ring.