For the preservation of the page itself. All web pages are vulnerable to manipulation, seizure or outright DDoS/deletion.. Makes it easier as well if you want to archive it for yourself. Works for keeping people honest and for capturing data if access to it somehow gets lost.

Also for some websites (such as MSM), it doesn't generate a hit on their website, which would allow them and their advertisers to get paid. In this case it is more for preservation though with the information being volatile