Fans that look like this:
Is it to somehow help the balance of the engine?
It isn't something restricted to old vehicles; my Lumina has fans like this, though not as irregular as the picture shown in the question.
As far as I can recall, the chief reason cited for this by the manufacturer is noise reduction. You'll notice the additional weight on some of the blades to ensure that rotational balance is maintained despite the asymmetry.
It may be to allow a hand/tool to access a screw or bolt that would otherwise be inaccessible if there wasn't one opening between the fan blades large enough. Although I can't think of how that would apply to electric fans