Brother Brigham and all those wives... Why?

Answer 1

One of the reasons I heard from the granddaughter of a polygamist (who must've been 80+) was that women survived the trek across the plains more often than the men. Since there was a surplus of women, men who were financially able to take another wife were "commanded" to support another one and any kids.
Another woman at the same church meeting and about the same age said that her mother was the daughter of a polygamist and her and her soon to be husband and a few friends were all laughing about polygamy (I'm betting this is in the 1920s or 1930s) and her mom came into the room and said, "there's no more polygamy because we're not worthy of it!"
So there you have it...genuine Mormon polygamy tales from Utah.

Answer 2

Why did lots of prophets in the bible have multiple wives?
Part of the reason polygamy was practiced in the first place was because there were more women in the church than men. We believe you must be married in the temple to attain the Celestial glory. Now how could all those women receive those blessings if they were never married? They couldn't. I know lots of people say "oh they just wanted to have sex with lots of women". Well, they will be judged for thier works in life as will you, you will never know what their intentions were, if they were following what the Lord had commanded or just horny.
Also, even now in the church, men can be sealed to more than one woman. It's doctrine, I don't question it.
Men will probably have several wives in the Celestial Kingdom, not necessarily every man, but some will. It is very possible that our Heavenly Father has more than one wife. But that I don't know for sure of course.

Doctrine & Covenants 132
37 Abraham received concubines, and they bore him children; and it was accounted unto him for righteousness, because they were given unto him, and he abode in my law; as Isaac also and Jacob did none other things than that which they were commanded; and because they did none other things than that which they were commanded, they have entered into their exaltation, according to the promises, and sit upon thrones, and are not angels but are gods.
38 David also received many wives and concubines, and also Solomon and Moses my servants, as also many others of my servants, from the beginning of creation until this time; and in nothing did they sin save in those things which they received not of me.
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52 And let mine handmaid, Emma Smith, receive all those that have been given unto my servant Joseph, and who are virtuous and pure before me; and those who are not pure, and have said they were pure, shall be destroyed, saith the Lord God.
• • •
61 And again, as pertaining to the law of the priesthood—if any man espouse a virgin, and desire to espouse another, and the first give her consent, and if he espouse the second, and they are virgins, and have vowed to no other man, then is he justified; he cannot commit adultery for they are given unto him; for he cannot commit adultery with that that belongeth unto him and to no one else.
62 And if he have ten virgins given unto him by this law, he cannot commit adultery, for they belong to him, and they are given unto him; therefore is he justified.

Of course it was not practiced after a certain time.