?
We do that to remove the snow that gets stuck in the classic skis due to the grip wax (which is kind of a very sticky glue).
What happens is: we (skiers) have to stay the more we can just inside the classic track, because there the snow is compact, so it doesn't get stuck in the grip wax. When we step outside, even if it's just for 1 second, the snow (in this case, in flakes, because outside of the track) can get picked by the glue. If that happens and the snow stays in the glue, apart from losing the glide of the skis, you can also fall because the skis brake suddenly while you are trying to glide (like some jolts) so the skiers have no other option than stop and clean it up.
Sometimes, that can happen even if you stay all the time inside the track (you know, those two lines). It happens when it's snowing a lot, for example.
You can see that before the start of a classic race, the skiers move their legs a lot. It's not because of nervousness, adrenaline, etc, but because if the skis stay still while you are standing on it, the snow can get stuck under the skis the same way. ?