the finish line has to be in front of race control so that, in the event of a total systems failure, they can determine the order of the grid by eye.For some weird reason which I now can't remember, some tracks have a different start and finish line.
i assume they dont put the start line at that point on some tracks (such as silverstone) because there wouldnt be enough room for the grid without backing them onto a corner that would be too dangerous to start on