As a jumper you can't always see exactly how long your jump is going to be, e.g. if there's snow...
It's just an addition to the normal lines, and the use of the V-look is that it's easier visible, because the wider the difference between the two lines, the longer the jump...
I don't know if it's necessary in DSJ, but I hope i could explain it a bit...
Thank you for good explanation Severin!
For dsj only to add realistic look then. I watched the tv pic's from Oberstdorf today, and saw the lines. But they were not red there, they were definately marked simply with sprigs of spruce just like those 5m lines. So if the point is to make things look like in reality, red lines is obviously not essential. I think that if we should have the lines, they should look rather discrete in order not to be disturbing.