Union by rank is a technique used in data structures, particularly in the implementation of disjoint-set or union-find algorithms. This method helps to keep the tree representing the sets as flat as possible, which improves the efficiency of operations like union and find. When two sets are merged, the root of the smaller tree is attached to the root of the larger tree, minimizing the overall height of the resulting tree. This approach is beneficial because it reduces the time complexity of operations. By keeping the trees balanced, union by rank ensures that the depth of any node remains logarithmic relative to the number of elements, leading to faster queries and updates in the data structure.