First, in earlier times, wars and battles often decimated male populations leaving widows and their children. It was commonplace for the conquerers/victors to marry the widows to provide for their welfare. In general terms, marriage was often a charity in itself. For example, Hasan marries Fatimah because it is unlikely that anybody else would marry her. Harun may have also married Mariam for similar reasons.
Second, many wives led to accumulation of wealth as women had fortunes and properties that would be transferred to their husband's fortune/name. Similarily, having many wives may have also indicated manliness and wealth on the husband's part as he could support many wives and children.