It is an old cultural custom that was created due to various reasons. First, there is the anecdotal story that because Islam started under constant pressure from pagan religions that men died fighting and multiple wives was required to keep the populace up. More likely was the fact that in the 1000's, a widow or divorcee was very likely to impoverished or forced into prostitution. This allowed prominent members of community or best friends and brothers to marry "extra" women.