Yibam (levirate marriage): A wife is considered as one of her husband's limbs like Eve was of Adam

Chinuch (598): The basis of the mitzva of Yibam (levirate marriage) is that a woman through marriage age becomes like one of her husband's limbs. That is because nature necessitates this as the result of the story of the first father (Adam) from whom was taken one of his limbs and from it G-d created a woman. Therefore this man who died without children who would have been a portion from him for his memory and who would have taken his place in the world to serve his Creator. He has left no remnant of himself in the physical world except for his wife who is the bone of his bones and the flesh of his flesh. Consequently it is a manifestation of G-d’s kindness to him to establish offspring for him from his wife through his brother (who is also like half his flesh) in order that there shall be offspring for him to replace him in the world and to serve his Creator in his stead. Therefore he will receive merit through this offspring in the world of souls where he is now. As is known a son brings merit to his father (Sanhedrin 104a), “A son brings merit to his father but a father does not bring merit to his son.” Similarly in truth, the living brother who fathered the children from the wife of the deceased also receives a portion of their merit. However not all the merit goes to him, because his deceased brother takes his portion because of the great portion that he has in the children i.e., the wife that was part of the deceased brother’s original portion as I have explained. And according to this, we can understand the meaning of Bereishis (38:9), “And Onan knew that the offspring from his deceased brother’s wife would not be his.” This means that he knew that not all the merit of the offspring of his deceased brother’s wife would belong to him because his deceased brother would take some. Onan simply was not interested in receiving less than the full merit. And also because perhaps the main merit would go to his deceased brother who was like the owner of the field while he was only be like the sharecropper. (It is well known that sometimes the sharecropper must also supply the seed.) This idea was expressed in Yevamos (22a), that as long as the deceased brother has any remnant in the world – whether it is a son or daughter from another woman or even if it a son or daughter who is a mamzer – the wife of the deceased is exempt from Yibum. Thus it seems that the basis for Yibum is only to memorialize the deceased brother’s name and to give him a portion and merit in this physical world. [See Moreh Nevuchim 3:49]