Why fold in ingredients, why not just whisk?
Folding is almost always done when you have one ingredient like whipped cream, egg whites, meringue, or similar which has had a lot of air whipped into it, and you are incorporating that with another ingredient.

The folding motion is meant to disturb the whipped ingredient is little as possible, in order to retain the whipped in air, and thus the volume of the mixture.  If you simply whisk, or beat, the air would be beaten out.

Whipping the egg whites or whipped cream alone forms a foam with nothing but the air and the fat in whipped cream, or the protein network in egg whites.   It us unstable much like the bubbles we used to blow as children.

If you add another ingredient, like flour, it can act as an abrasive, popping the little bubbles and deflating the foam.    Ingredients which can dissolve fully, and so won't be abrasive to the buddies, can still dissolve into the foam walls, and change their chemistry to the point where they don't hold air any more, again collapsing them.  So even though the motion still might be called whisking or whipping, the result is not the same.  

There are only a few ingredients, such as sugar for whipped cream or egg whites, that can dissolve into the foam walls and not disrupt it, and these ingredients are best incorporated in the original whipping phase where the air is incorporated.

Even with folding, there is some volume loss; the goal is to minimize it.