The trim of the rig is one of the biggest safety issues aboard any boat. The failure of fittings, shrouds or stays can damage your boat, break your mast or even worse hurt someone. To avoid accidents it is of biggest importance that the rig is set up correctly. Too little tension on the shrouds, makes the leeward shroud slack. In a manoeuvre or even in waves the slack is abruptly strained, which is far more pressure on the shroud than if it was properly tightened.
A rather less existing problem is too much tension on the shrouds. Too much tension can cause a permanent stretch in the shrouds or even break your mast.