It’s raining out and the road your on starts going up a hill. Theres a lot of water on the road so you’re not driving fast. An automatic sees your rpms dropping because you need more power to go up the hill. It doesn’t know its raining. It downshifts to give you more power to get up the hill. You went from 50 mph at 1300rpms to 45 mph at 5000 rpms. All that power now going to your tires creates more opportunity for your wheels to loose traction in the rain and fishtail.

In a manual you put in a gear that keeps your rpms high enough to maintain speed but not 5000 rpms to “go faster”. The power to the wheels stays exactly where you want it to be based on the road conditions. Replace with snow, sleet, etc etc.