Because you must not block the traffic from the right. It's the same principle as on an "ordinary" crossings where you have to stop before the green light if you can't get across, just in this case the white line's location's shifted.
If there's a queue ahead of the red light and you can't get across the "crossing" you have to stop for the white line. Otherwise you block those from the right who want to turn left, which you must not do.
Then you stop in front of the line if people want to cross the street, if nobody wants to cross the street then you can just drive on without stopping.
Far as I know, the white line across the lane marks where you must stop, i.e. where your front bumper should not surpass, should any of a number of conditions be present. If you drive over that line during any such condition you may incur a fine or you may incur liability should you cause an accident. I don't think it has any other legal purpose than demarkation.
If the street in front of you up to the red-light is free, you don't stop even if someone wants to enter (because you are on the "Hauptstrasse" - the road with way). If the street ahead of you is blocked you stop behind that line - whether or not someone wants to enter at the moment you arrive there.
Or simpler: You just do not stop where you block entrance and/or people wanting to turn left accross your path
But this is the case before "any" pedestrian crossing (at least those without a traffic light). No? So what's the point of solid line? That's exactly my ambiguity.
If the light is off or flashing yellow then if there are any pedestrians waiting to cross the road you stop behind the white line to let them cross. If not, then you don’t need to stop.
You have read the posting to which i reacted? (There's no red light in that situation, or other roads that cross, there's only a pedestrian crossing and we stop for those people that want to cross unless a green light tells us otherwise.)
Your question has already been answered, what more do you want to know?
To say it with other words: The first couple of meters after such stripe is a forbidden to stop zone, this to prevent other roads, exits, pedestrian crossings, bicycle paths, tram lanes or whatever you can think of from being blocked.
And you only stop if you must stop for something, you don't stop for the sole reason there's a line on the road.
You can also regard the white line as establishing a buffer for the safety of pedestrians or other vehicles, or even keep you from getting your snoot smashed off by a rocketing tram.