King A has Queen B, Consort C, D, E.
1) Queen B is from the most influential noble family in the kingdom. By marrying Queen B, the King can extend his influence to the noble and ensure they are in check. The noble will tend to align with the most influential noble family. Queen B being part of that family shows align with the royal and the nobles.
2) Concert C is from Country C-1. C-1 is well known from its incredible resources and minerals. By marrying Concert C, Country decided to offer privileged access to some resource to the Kingdom and better term deals.
3) Country D was eying to start a war with the Kingdom. By using Consort D has a political hostage, the Kingdom is ensuring that Country D stays in check. The Consort D is the token of the willingness of Country D to remain in good terms with the Kingdom. Consort D serves a peace token.
4) Consort E is well know for her unparalleled insights and quirks. By Marrying E, the Kingdom benefits from the talents well kept by the Consort.
Just see politics as a game of "I get what I need. I get what I want. You do this for me, I do this for you. I get what I need. I get what I want.".
Well... The women are being used as bargaining chips so thankfully that shit doesn't happen anymore. Well in most countries at least...
Im not a fan of harems , i dont understand how politics works