its a simple process of merge in turn, we have a road near us that goes from dual to single carrageway, if everyone sat in the left then it would cause issues on the Roundabout a couple of hundred yards behind, using both lanes, means twice as many cars can que before it spills onto that round about, and we all know idiots don't keep round abouts clear.
If you have 2 lanes of 25 cars, or one lane of 50 cars, it's going to take just the same time for the que to go down, IF people take notice of the 'Merge in turn' road sign and follow the clear instruction .