Why is drum to disc called an upgrade?

[dmjlambert]

I'm just curious. Why is drum to disc brake conversion called an upgrade by so many people? Is it just because later model trucks happen to be made with disc brakes so people assume it is better? Has that ever been demonstrated? Did disc brakes come about because they were cheaper to make and less complicated for maintenance? Or because they are actually better in some other way? I know disc can cool down better if subjected to extreme use, so I suppose if I lived in the mountains there may be a slight advantage with disc. Anything else? I was just wondering if the upgrade idea is something that is not real but gets passed around, like having distributor gear engaged some number of teeth off that supposedly could affect timing.