It is done for some walls, but it's much less common. Part of the issue is that, yes, hot air rises, so a lot more moisture will find its way to the roof than

It is done for some walls, but it's much less common. Part of the issue is that, yes, hot air rises, so a lot more moisture will find its way to the roof than to the walls. Part of the issue is that roofs are (traditionally) sealed much more thoroughly than walls (because rain falls on roofs), so the moisture has a harder time getting out. The ventilation channels provide an egress for the moisture below the sealed roof surface. A big part of the issue, at least in many climates, is snowfall. If it's cold outside and warm inside the house, any snow accumulated on the roof is melted from beneath and not from above. As the slush and water try to run down the roof under the unmelted snow, ice dams can form that prevent effective drainage; water pools and leaks down through the roof. A ventilation channel under the roof surface keeps the surface at or near outdoor temperatures, so the heat of the house doesn't melt the snow from beneath.