I have seen at lot of photos of ski aircrafts, dragging a wire behind the tailwheel, but dont understand why? Anyone who can explain....

Just guessing here, but since you can't set your brakes, maybe it is to tie down while hand propping??

It's not a wire, it's a rope, or more appropriately a line. It offers a "handle" for someone else to get hold of to help a pilot turn around on skis in a tight spot.

MTV

Thanks, thats what i suposed!

When it's real slick you can toss it in front of your ski and helps to pivot or turn , then power over it and take off and you still have it when you get there.
GT

Yea, but dont destroy Your skies when turning around in i TO SMALL circle...