Sometimes, in the course of human events there are occasions when Witches come together en masse to ameliorate, that is, 'correct' or 'adjust' matters that have gone horribly awry. These would be times when insidious and ruinous evil is running amok and rending disturbances in the Force. I suppose one way to try to explain this is to have you picture a large wall tapestry that has an ever-expanding hole or stain in it. If left ‘unattended’, it would eventually rot from decay and cease to be. In order to ‘fix’ it, one would have to excise the old, raveled threads and then weave new threads into it.