Being hot brings blood to surface. Being cold sends it to your organs. Both extremes work your heart like a pump. That’s why the whole sit in the sauna, roll in the snow, sit in the sauna thing makes people glow.
But why is increased blood flow good for you?
Well, it helps flush your circulation for starters, pushing blood through all your capillaries, veins and arteries. It will exfoliate your skin and flush impurities from it, thus helping your complexion (firm-bodied women of all ages around pool sides say it stops cellulite). Evidence also demonstrates that your body adapts to the cold with repeated exposure and this may improve your circulation, particularly to your extremities – no bad thing in the winter months.
You could get these benefits by switching between the hot and cold taps in your shower (or the sauna, snow, sauna thing) but that doesn’t sound nearly as fun as quick dip in your local pond followed by wrapping up warm afterwards.