Ian Jackson presented the following explanation :
I don't like queues, do you?

No, but I can cope with them without becoming too upset.

When the traffic merges well before it
absolutely has to, there's a far better chance of it being able to maintain
speed, and no queue forms. If left until it has to merge, there is more
chance a bottleneck occurring, and everything grinding to a halt. And as I
said, I don't like queues!

The merge happens at full speed with a little cooperation and everyone
behaving at the point where it has to merge.

If everyone merges early, it is too much of a temptation for someone to
not go steaming down the outside lane, to bully there way in. That
causes things to grind to a halt, whilst extra space is made for the
intruder.