The main feature of the poles is to mark the position of the road when snow is covering the usual markers. You don't need the poles most times, but during the window between snowfall and snowplowing, especially if the snow is being blown into snowdrifts further obscuring the typical "road shape" you would have to look for it the poles weren't there, they're your only means of navigation, and can be used to judge the size of snow drifts.

The poles have a standard height, and a standard distance between poles.

In places with less snow it’s some times just a reflex on the ground rather than on a pole. I think those little matchbox ground reflexes are common in parts of the US at least?

Yes. It's not everywhere, but it's very common in regions with snowfall.