For example if you want to save a web page you're browsing, or a whole website, in a way that is a bit more reliable than hitting ctrl+s; and then want to search through those etc.
Web archival is a very, very complex and deep topic. If you're interested in it, the archiveteam's wiki is a good resource. Start here, I think:
https://wiki.archiveteam.org/index.php/Introduction
https://wiki.archiveteam.org/index.php/Software
https://wiki.archiveteam.org/index.php/The_WARC_Ecosystem