הלכה על סנהדרין 207:3

It is from the roots of the commandment [that it is] since the woman that a man marries, behold, she is like one of his limbs - as so does nature necessitate because of the story of the first father, as one of his ribs was taken and from it did God build woman - and since this man died with no sons, that there should be a portion from him for his memory and to fill his place in the world in the service of his Creator. And also there is no memory of him in the physical world besides this woman, who is the bone of his bones and the flesh of his flesh. And [so] it was from the kindnesses of God to him to establish seed from her for him, through his brother - who is also like half of his flesh - in order that this seed take his place and serve his Creator in his stead, and so that he merit through him in the world of souls, where he is. As it is known that a son gives merit to the father; as so did they, may their memory be blessed, say (Sanhedrin 104a), "A child gives merit to a father, a father does not give merit to a child." And truthfully likewise does the living brother that fathered the children from the levirate wife also get a share in them for himself, and he also gets merit in their merit. Nonetheless, not all of the merit will go to him; but rather his brother will take his portion because of the big portion that he has in them - and that is the wife that fell to his portion first, as we have said.