What is the author's likely purpose for the figurative language used in paragraph 6?

I assume you are referring to the paragraph beginning "Oh! But he was a tight-fisted hand at the grindstone, Scrooge!"
This paragraph is rich with imagery and figurative language, all of it serving to demonstrate not only how "hard and sharp as flint" Scrooge is, but also how actively his behavior and personality affect those around him. The string of active verbs applied to Scrooge make quite clear that his ungenerous nature has had direct effects upon those who work under and around him; Scrooge is "squeezing, wrenching, grasping, scraping, clutching, covetous." Dickens notes that "a generous fire" has never been struck from flint, even by steel—that is, Scrooge could not even be bullied into generosity by an equally hard person.
That Scrooge is locked up within himself we see from the comparison drawn to an oyster; there is also a semantic field of cold springing from the "icy chill" within Scrooge, which is literally visible in his "shrivelled cheek," "stiffened gait," and other elements. It was a popular Victorian concept that one's inner character would emerge in one's features, and Dickens plays with this idea here. Scrooge, the language of this paragraph shows, "carried his own low temperature about with him" and inflicts it upon others, even at Christmas.