Well... There are two directions to go with this answer. One is that there is a vacuum in a tube, so any heating of the elements inside must cool exclusively by radiation. There is no convection. The relatively large space between the glass and the elements means that the temperature of the glass, as it relates to being a mass that can absorb and radiate heat away from the tubes internal elements, is almost moot. So, WRT tube life as it relates to tube temperature, a fan does very little.
But... Then there's the many amps that actually DO benefit from having a fan. I've read here that the fan is there to cool everything else in the amp that is heated by the tubes. While this is also relevant it doesn't account for reports of extended tube life.
So on the one hand we have physics telling us it doesn't work. On the other hand we have builders and users doing it that tell us it does. It wouldn't be the first time real world experience and scientific explanation are at odds.