In the 1980's mountain bikes pretty well took over the market, and a lot of customers got reintroduced to cycling by getting a mountain bike, which they often rode on the roads. When those riders got interested in getting a road bike, they were used to wide handlebars and sloping top tubes, and suddenly these features began to be normal for road bikes. At this point it was discovered that older bikes have narrow handlebars, which was odd, because it hadn't previously been the case.

Narrow road bars hold a few advantages. Of minor gain is less material, arguably lighter weight.

However a large gain could be considered in lowering the frontal area, a more tucked in aero advantage. Wide bars make more of a parachute. Follow the principal of aero bars but also the old clip on super narrow drops. You will see obvious gains in speed.

Then again, they might not help in opening up the chest or breathing, or other issues during climbs. I've read and been told the standard is to find a bar width just under shoulder width. I say use whatever is comfortable.

You might give them a try first. If a rider is built like a mantis, they can ride the wide bars with their elbows in on their chest. But if you have more muscle/beef/fat on the body/frame, you can't do that. There is only so much room in that cube that surrounds the TT. Those quads, forearms, biceps, gut and lats start hitting each other. Narrow handlbars force the elbows out which provides some more room in the cube.