The purpose of these lines is to explain that beauty can fade. The lines suggest that beauty ("every fair") is subject to decline ("sometime declines"), either by chance or through the natural changes of time ("nature's changing course untrimm'd"). The beauty represented in this sense is transient and will eventually fade, teaching the reader a lesson about the无常 of life and the inevitability of change.
As with any endeavor, the pursuit of knowledge, understanding, and appreciation of beauty comes with an understanding that it too is subject to the natural cycles of decay and renewal. This understanding, in turn, can provide a deeper appreciation for the fleeting nature of beauty and the importance of treasuring and enjoying the moment.
Furthermore, the lines invite the reader to contemplate the broader implications of the theme of beauty's decline. This can include discussions on the role of change in nature, the impermanence of human achievements, and the wisdom of embracing the known in favor of the unknown., the purpose of these lines is to use the concept of beauty's decline as a starting point for exploration and introspection. Through this lens, readers can gain a deeper understanding of the world around them and the nature of existence itself.
