A more serious reason to know who is hosting a website is if someone has stolen content from your webpages. If you’re looking into suing the person that committed the theft, you’d have to know who the host is even before you file a complaint with the DMCA.

Also, websites that have many spammers making comments, input malware or spyware, or even attack your computer should be reported. But reported to who? It’s their host companies that can get them off the net, so you’ll need to find out who they are.