This technique is preferred for trees with several trunks, or with weaknesses or malformations. Not only does it prolong the life of the tree, it also helps prevent accidents. If implemented early enough, it can in some cases avoid the need for pruning or felling.
Whether it's because of problems in the very structure of a mature tree, or because the branches or trunk have suffered significant damage following a storm, the tree may need a helping hand to preserve its own balance or prevent accidents.