In Mary Shelley's Frankenstein, chapter twelve offers a very human side of Victor's monster. Prior to the chapters which offer the perspective of the monster in the text, the monster has only been characterized through Victor's eyes.
Beginning in chapter eleven, the monster begins to tell his tale. He speaks of his initial recollection of his existence, the fears he had, and of the travels which led him to the De Lacey's home. It is upon the monster's arrival at the De Lacey's home where many readers begin to sympathize with the monster.
What chiefly struck me was the gentle manners of these people; and I longed to join them, but dared not. I remembered too well the treatment I had suffered the night before from the barbarous villagers, and resolved, whatever course of conduct I might hereafter think it right to pursue, that for the present I would remain quietly in my hovel, watching, and endeavouring to discover the motives which influenced their actions.
After reading this passage, readers to to see a softer side of the monster. He is not what Victor has portrayed him as. Instead, he recognizes the impact the family has upon him when he states the following:
This trait of kindness moved me sensibly. I had been accustomed, during the night, to steal a part of their store for my own consumption; but when I found that in doing this I inflicted pain on the cottagers, I abstained, and satisfied myself with berries, nuts, and roots, which I gathered from a neighbouring wood.
Feeling a strong tie to the De Lacey's, the monster begins to feel sympathy for their suffering. He finds it necessary to help them (and not ask for anything in return). For the most part, the monster learns about the importance of love, family, and true heart-felt emotion.
Therefore, the purpose of chapter twelve is to allow the reader to connect with the monster on a human level, to allow the reader to sympathize, and to allow the reader to hear the reasons for how he came to be (from his point of view).
(On a side note, the link below offers the chapter summary to the eNotes chapters which speak to the chapter in question. While not denoted by "Chapter 12," the link is the correct one which aligns with the text in question.)