That's one reason. Another is to encourage members to think about the picture and the story more than perhaps they do with the 75-worder, so instead of posting within two minutes of the Challenge going up** they spend some honing their writing, which is after all the point of being an Aspiring Writer. With a proper prize in the offing, it's more prestigious than the 75-worder, and therefore deserves rather more thought. (Whether it gets it is another matter, of course!)
** and then spending the rest of the month moaning about the changes they should have made...
As if we would. I quite like the rule, and usually potter about with it for a day or two and then review just before I post. What I try not to do is write a second one, or even think of a second idea, otherwise I'd just tear myself apart choosing.