God explains exactly why here:
http://LDS.org/scriptures/dc-testament/ ... 6?lang=eng1 Verily, thus saith the Lord unto you my servant Joseph, that inasmuch as you have inquired of my hand to know and understand wherein I, the Lord, justified my servants Abraham, Isaac, and Jacob, as also Moses, David and Solomon, my servants, as touching the principle and doctrine of their having many wives and concubines—
2 Behold, and lo, I am the Lord thy God, and will answer thee as touching this matter.
3 Therefore, prepare thy heart to receive and obey the instructions which I am about to give unto you; for all those who have this law revealed unto them must obey the same.
Section 132 also makes it clear that it's a man thing.
Men can have wives.
Women can have a husband.
Here we go, here's the answer, clearly and explicitly stated by God himself directly to Joseph.
for they are given unto him to multiply and replenish the earth, according to my commandment, and to fulfil the promise which was given by my Father before the foundation of the world, and for their exaltation in the eternal worlds, that they may bear the souls of men; for herein is the work of my Father continued, that he may be glorified.
So God was clear about it, it was for producing offspring.
Now how did Joseph do on fulfilling the specific reason he was commanded to practice it?