You don't have to take my word for it: here's FLDS escapee Carolyn Jessop on the conditions under which she lived as church elder Merril Jessop's fourth wife:
She had to remain "of value," or else, presumably, her husband would tire of her and neglect her and her children. That's not the dynamic of a loving and mutual relationship that happens to include more than two people, that's the dynamic of master and slave.
To protect myself, I had to remain of value. Sex is the only currency – every polygamist wife knows that. A woman who possesses high sexual status with her husband has more power than his other wives. (emphasis mine)
She also has more children, and children are an insurance policy. Even if her husband takes a new and younger wife, a woman who produces a bevy of beautiful babies will earn respect.