What does the word "but" in line 9 emphasize in Sonnet 18?

The ninth line of a sonnet is traditionally what's called a volta, or turn. In the previous eight lines—the octet—the poet presents us with a problem, which is then solved in the following six lines, or sextet.
In the first eight lines of Sonnet 18, the speaker presents us with a problem: namely, how difficult it is to compare his lover to a summer's day. There are all kinds of reasons why this may not be the best comparison. Summers are often short and hot, for example, and everything that's fair in nature will eventually decline.
In the remaining six lines of the poem, starting with the word "But" in the ninth line, the speaker solves his problem by rejecting the comparison. Instead, he insists that his lover, unlike a summer's day, will live on forever, immortalized as he will be by this famous sonnet.
https://www.poetryfoundation.org/poems/45087/sonnet-18-shall-i-compare-thee-to-a-summers-day