If they were intake fans, I presume replacing the air every 4 minutes would lead to a significant temperature reduction, as I guess the air would not heat up much under the glass in that time.
However, as circulation fans, even with windows and doors open, there is a surprisingly large temperature difference between inside and outside (15 C would be unexceptional).
The volume of air in a 6' x 8' x 5' (guessing average height) greenhouse is about 7 cubic metres. A "gaming" PC fan I found online was rated at about 100 cubic metres per hour, which gives a pretty similar time for moving the entire volume of air in the greenhouse.
So, to my surprise, a computer fan could be just as effective in a small greenhouse as my much larger fans, in terms of fraction of the greenhouse's air moved in a certain time.
Gut feelings can be just as wrong as other poeple's beliefs, if not more so!
Anyway, I think Mike has it right, that the benefit temperature-wise is that mixing prevents pockets of stagnant hot air which can scorch plants.
