When is a man required to observe an 'iddah (waiting period)? And what is the wisdom behind that?

The waiting period (iddah) is specific to women and is not obligatory for men. However, a man is prohibited from marrying a fifth wife until the iddah of his fourth divorced wife ends. Similarly, he is prohibited from marrying his wife's sister, paternal aunt, or maternal aunt until her iddah ends. This prohibition is not called "iddah" in a legal or linguistic sense; rather, it is a figurative use of the term.