Adam Clarke on Genesis 16:2

Go in unto my maid - It must not be forgotten that female slaves constituted a part of the private patrimony or possessions of a wife, and that she had a right, according to the usages of those times, to dispose of them as she pleased, the husband having no authority in the case.

I may obtain children by her - The slave being the absolute property of the mistress, not only her person, but the fruits of her labor, with all her children, were her owner's property also.

The children, therefore, which were born of the slave, were considered as the children of the mistress. It was on this ground that Sarai gave her slave to Abram; and we find, what must necessarily be the consequence in all cases of polygamy, that strifes and contentions took place.