Um, perhaps to attach a lock cable for people who haven't figured out that saddle rails are convenient for this?

To hang your bike by when weighing it?

Or perhaps to tow children on go-karts with your bike?

Or perhaps to dangle things onto your back tyre to make "moto" noises?!

Any more sensible suggestions?

Sheesh, they don't expect me to sit on that do they?

I agree Shaun, I've got six of them plus a gel version (which gets soggy and feels hot) and an embroidered one that wears out my shorts (and gets soggy). 

I've been using the new Flite gel flow for a few months and though it looks a bit weird at first and feels like a bit is missing at the back when you sit on it on it I actually find it one of the comfiest saddles I've used so far.

Hole or no hole, the dropped pointy bit at the back must surely be a hindrance.  If you've had to get over the back of the saddle for steep bit, it will just hook your shorts when you slide back forwards.

And the hole is clearly just the right size and shape to store a single chocolate brazil nut for emergencies.