Cardio may seem like a pain in the backside, but it does several things for you:
It helps cardio-vascular health (surprise surprise

)
It clears away lactic acid - preventing muscle soreness.
It increases vascularisation - more capillaries mean more blood to the muscles, which in turn means that when you hit the weights, you get a better pump, and your muscles can recover faster.
You don't need to go mad - 3 lots of 20 minutes a week should suffice.