Jamieson, Fausset and Brown on Genesis 16:3

3. Sarai ... gave her to ... Abram to be his wife--"Wife" is here used to describe an inferior, though not degrading, relation, in countries where polygamy prevails. In the case of these female slaves, who are the personal property of his lady, being purchased before her marriage or given as a special present to her, no one can become the husband's secondary wife without her mistress consent or permission. This usage seems to have prevailed in patriarchal times; and Hagar, Sarai's slave, of whom she had the entire right of disposing, was given by her mistress' spontaneous offer, to be the secondary wife of Abram, in the hope of obtaining the long-looked-for heir. It was a wrong step--indicating a want of simple reliance on God--and Sarai was the first to reap the bitter fruits of her device.