There are two aspects to this question:
Peter uses this quote to shore up the contrast he recorded in 1 Peter 1:23 between the perishable nature of human seed and the imperishable nature of divine seed. He finds confirmation for that contrast in this quotation from Isaiah 40:6–8.
The context of this quote within Isaiah 40 serves to illustrate just how imperishable—and indeed powerful—the word of God (and the God of the word) is in contrast to the finiteness of man. The chapter portrays Israel as broken and helpless (Isaiah 40:1–2, Isaiah 40:11, Isaiah 40:27, Isaiah 40:29–31) and depicts also their oppressors as temporary and weak (Isaiah 40:23–24). At the same time the chapter presents the Lord God in his redeeming power and grace, almighty to save (Isaiah 40:10, Isaiah 40:12–14, Isaiah 40:21–22, Isaiah 40:25–26).
Using this quote as his vehicle, Peter affirms for his readers the radical difference that exists between God and man. In their pre-birth condition, they would not have tuned in to the significance and comfort of that contrast. Now that they have been born again (of divine seed!), they are in a position to appreciate the opportunities that come with being loved by such a God!
24 for “All flesh is like grass and all its glory like the flower of grass. The grass withers, and the flower falls,