Physical transformation becomes a gauge by which we judge our protagonist’s increasing reputation. As Buck gets stronger, faster, and more dominant, so his reputation grows and improves. There seems to be no division between men and dogs here, as both end up in awe of Buck’s prowess. Reputation is also a tool by which we discern which qualities are important in the world of the novel. Buck gets famous for pulling a staggeringly heavy load, so strength is important. Because he is known for his commitment to Thornton, loyalty is important. Reputation is then not only a noticeable feature of the protagonist, but also a tool for the readers.
While many elements contribute to Buck's value and worth, his physicality is the most important.
Buck's value is greater in the wild than it ever could have been had he stayed on the ranch on California.