The stall angle of a wing in ground effect reduces in comparison to the stall angle of a wing flying outside of ground effect. Doesn't this make soft field takeoff dangerous?
The decrease in stall AOA is more than offset by a decrease in the AOA required to achieve the same total lift and the result is indicated stall speed actually drops a little bit. And pilots generally don't fly AOA itself as a parameter, but pitch and speed.
But the main thing is the power required to fly is less in ground effect so there is more power available to accelerate. Normally when taking off you rotate and start right up because even though you are initially below best climb performance speed, there is ample reserve and you accelerate to it with no problem. Problem is in marginal conditions, where the airplane won't climb at all unless it's at its most efficient climb speed, you need to accelerate somehow without trying to climb, and you can't just roll along the ground faster, so you take advantage of the power bonus you get in ground effect.
Ground effect is much stronger the higher the aspect ratio. How strong? Watch this glider's L/D go through the roof: