A fatty up front and a skinny in the rear (fnar fnar) raises the front end and lowers the rear. This relaxes the head angle oh so slightly. And lowers the gearing a tad too.
Now whether or not any of us can actually feel the difference, well that's another thing all together.
Personally I'd say it was just for looks.
Back in 83 when I was only doing kickturns, I believe I felt a difference. I never ran a fat front tyre ever coz it seemed to steer weird, but rear tyres were all 1.75s as thats the only size shopper tyres came in.....