One explanation that's often given is that having the fan, or fans, in front of the rad gets in the way of the airflow to the otherwise unobstructed front surface. Fan/s in front breaks up the flow and rather randomly and variably force the air through the core. When fitted on the back, if you have the room and are prepared to make up some often difficult brackets, it just accelerates the natural air flow through the rad. Sounds a reasonable theory. As you only really need the fan when going slow and there isn't much airflow anyway it likely won't make much difference. A shroud on the back does help the fan to pull air through the whole core and not just the area in front of it.
It's certainly quick, neat, easy and out of the way of your fingers when it clicks on suddenly while poking around the engine bay, to mount a fan front of the rad in an Amazon. I'm not keen on the through rad plastic tie wrap type mounts as I'd be worried that they could fret their way through the tubes over time, especially on a old rad that might be getting a bit frail.