What benefits does a car get from a semi-automatic transmision?

I have a semi-auto and MPGs are pretty much the same. Not the same as manual because there is no clutch pedal and the transmission lever you just switch - or +. It's ok but I just stay in automatic. Once in a while switch to manual for fun. Well, I also heard that in real icy conditions you can also use it to downshift so the engine slows you down if going downhill to use along with brakes to avoid slippage, but double check that with a dealer, I haven't tried it.