Why they are important.
The whole point of a Yellow box junction is to keep a junction clear in the case of this one so that traffic can turn into the side road without holding the traffic behind it up. If the traffic coming the other way is held up will block the junction and this will lead to a “knock-on” effect causing traffic to back up all around the area.
Box junctions work in that respect as long as people respect them.
However in the case of this clip the car driver didn’t respect the rules which are quite simple “Do not enter the yellow box unless your exit is clear”.
The silver car didn’t follow this rule and got trapped on the box, in other places the boxes are monitored by cameras and you will get a £130 fine for blocking a box junction.
The fine is high because of the chaos a selfish driver who blocks a box can cause, all you have to do is watch the clip to see the issues that can be caused.