The last three days I have been hardening melon and sweet pepper plants before transplant. I do it by locating them in the deep shade of lilacs the first day, second day I move them halfway out of the shade, and third day almost completely out of the shade. On the fourth day I can transplant to the garden, so I did today.
If I didn’t harden off the plants, but transplanted directly from the greenhouse to open ground, the plants would get sunburned on the leaves.
Plants need time to adjust to sun and wind, just as we people also need time to be able to tolerate the sun. If the adjustment take place over some days the leaves have time they need to build a more tough leave surface, just as we people need a little time to develop a protecting sun tan.