Here's how I understand it. They change to get around people of
course. They also change to see if the snow is faster in another lane -
and might change back or try another if not. Another reason, I've been
told, is that when a classic skier changes lanes there are allowed a
certain amount of skating strides - necessary to change lanes. I don't
know if this is an actual advantage, merely mitigates the effort
involved, or has no effect.