I think it was more a purely faith-based principle, but it probably had some practical benefits in the 1800s.
And I agree there will be some Mormons(even back then) that will abuse the practice such that it fits stereotypes at times. But in general, it was a practice of the Mormon church that was not evil, although the perception to others can be bad. Certainly today’s apostates in Arizona are turning it into a tragic legacy. But the main leaders of the church(Joseph Smith and Brigham Young) were men like the prophet Abraham..(in my opinion).
If somebody tried that with today's women, there would be a noisy multiple homicide started by one of the women.
THE DOCTRINE AND COVENANTS
SECTION 132:34 God commanded Abraham, and Sarah gave Hagar to Abraham to wife. And why did she do it? Because this was the law; and from Hagar sprang many people. This, therefore, was fulfilling, among other things, the promises.
NIV Genesis 15:1-6
1. After this, the word of the LORD came to Abram in a vision: "Do not be afraid, Abram. I am your shield, your very great reward. "
2. But Abram said, "O Sovereign LORD, what can you give me since I remain childless and the one who will inherit my estate is Eliezer of Damascus?"
3. And Abram said, "You have given me no children; so a servant in my household will be my heir."
4. Then the word of the LORD came to him: "This man will not be your heir, but a son coming from your own body will be your heir."
5. He took him outside and said, "Look up at the heavens and count the stars--if indeed you can count them." Then he said to him, "So shall your offspring be."
6. Abram believed the LORD, and he credited it to him as righteousness.
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.NIV Genesis 16:1-16
1. Now Sarai, Abram's wife, had borne him no children. But she had an Egyptian maidservant named Hagar;
2. so she said to Abram, "The LORD has kept me from having children. Go, sleep with my maidservant; perhaps I can build a family through her." Abram agreed to what Sarai said.
3. So after Abram had been living in Canaan ten years, Sarai his wife took her Egyptian maidservant Hagar and gave her to her husband to be his wife.
4. He slept with Hagar, and she conceived. When she knew she was pregnant, she began to despise her mistress.
5. Then Sarai said to Abram, "You are responsible for the wrong I am suffering. I put my servant in your arms, and now that she knows she is pregnant, she despises me. May the LORD judge between you and me."
6. "Your servant is in your hands," Abram said. "Do with her whatever you think best." Then Sarai mistreated Hagar; so she fled from her.
7. The angel of the LORD found Hagar near a spring in the desert; it was the spring that is beside the road to Shur.
8. And he said, "Hagar, servant of Sarai, where have you come from, and where are you going?" "I'm running away from my mistress Sarai," she answered.
9. Then the angel of the LORD told her, "Go back to your mistress and submit to her."
10. The angel added, "I will so increase your descendants that they will be too numerous to count."
11. The angel of the LORD also said to her: "You are now with child and you will have a son. You shall name him Ishmael, for the LORD has heard of your misery.
12. He will be a wild donkey of a man; his hand will be against everyone and everyone's hand against him, and he will live in hostility toward all his brothers."
13. She gave this name to the LORD who spoke to her: "You are the God who sees me," for she said, "I have now seen the One who sees me."
14. That is why the well was called Beer Lahai Roi ; it is still there, between Kadesh and Bered.
15. So Hagar bore Abram a son, and Abram gave the name Ishmael to the son she had borne.
16. Abram was eighty-six years old when Hagar bore him Ishmael.