OK, in the old days pedals and toe clips, you had cycling shoes mostly with thick leather soles on to which were nailed metal plates with a groove in them that matched up to the pedal, when riding with these if you wanted the best efficiency you would do up the toe straps so that your foot would not come out of the pedal. There was no rotation of the foot in the pedal and no way of removing your foot unless you undid the strap first. Most days would see your foot going to sleep due to the strap cutting off circulation to the toes.
Now we have adjustable pedals where you control how much effort is needed to release the foot, most of these pedals have a built in angle of rotation so your foot can move a bit, and release is easy in one or both directions. On top of this your foot does not go to sleep as it is not binded to the pedal with a toes strap (alfredo binda were the best).
If anybody here new to spd is having trouble releasing their feet just adjust the spring load as explained earlier, in the end if you loosen them too much you want be able to stop your foot coming out and this would be almost a danger, especially out of the saddle.
Voila, and on top of that your sole is much closer to the level of the axle of the pedal so your COG is lower, better bike handling.