what practical purpose does the beautiful stranger serve?

http://www.gradesaver.com/frankenstein/q-and-a/what-practical-purpose-does-the-beautiful-stranger-serve-63141/

It is important to realise how Shelley uses the chapters that the monster spends with the De Laceys to present him and us with the companionship, warmth and love that is part of human nature. Of course, this is a mixed blessing for the creature, as he realises that he can never be part of these blessings. However, also, these chapters perform a much more practical purpose, explaining how the creature learnt to read and speak. The arrival of the beautiful stranger, who turns out to be Safie, provides an excellent opportunity for the creature to eavesdrop on the lessons that Felix gives her. He learns not only to read but also the basics of history, politics and religion of mankind.

"My days were spent in close attention, that I might more speedily master the language... While I improved in speech, I also learned the science of letters as it was taught to the stranger and this opened before me a wide field for wonder and delight."

The arrival of Safie and her subsequent tutoring thus allows the creature himself to be tutored about mankind, and a race that profoundly puzzles the creature, as mankind is "at once so powerful, so virtuous and magnificent, yet so vicious and base." Note how this feeds into the theme of the confusion between man and monster that dominates the novel.