well, I heard that you do that to prevent slugs from eating your plant because they dont like going over sharp edges.
But, a better technique for deterring slugs (if an outdoor grower like me) is to get an empty tin of beans or something, dig a hole for it in the ground next to your plant and then fill it half-way with beer. The slugs love it and they drown once they're in, but the plant remains unharmed. I would guess that they die pretty happily too with all that beer in their system! haha

peace