The Book of Mormon identifies one reason for God to command it: to increase the number of children born in the gospel covenant in order to “raise up seed unto [the Lord].” (See here.)
God declared in the Book of Mormon that monogamy was the standard; at times, however, He commanded plural marriage so His people could “raise up seed unto [Him].” (See here.)
Church members viewed plural marriage as a commandment from God, an imperative that helped “raise up” a righteous posterity unto the Lord. (See here.)
So the answer is: BABIES FOR JESUS! I'm not sure why God needs babies so badly when there are plenty being born all the time. What makes children born "in the covenant" so much more valuable than all the others. Shit, why's he need children at all? They don't even make money to pay tithing. Are children the only people who believe in god stories anymore? No missionary work? Just baby-making sex? Can't God encourage some inter-tribal adoption?
"Hold on a second, sugar, let's let the pregnant ones catch up."
If God needed babies so badly why didn't Adam get a few hundred wives instead of just little old Evie? Why didn't Noah's boys fill the Ark with God's most fertile? Why didn't Nephi and his brothers have a shit ton of sister wives? Why didn't Jesus' mighty twelve just hang around in harems all day instead of going out and preaching? Why didn't Joseph have any children with his 30 plus wives? Why did Brigham only have a couple more children than he had wives? Why do we see that polygamy fails to produce more children than monogamy?
Does God suck at math? Or was Jacob wrong about the divine purpose of polygamy?
At least Lot had two daughters to rape him instead of just one.