Imagine if for websites you had to get a brand new domain if you ever wanted to change the contents of your web page. You can't just go to google.com because it would be immutable. You would have to somehow know you have to go to google10393928.com and any links to google on the internet would be linking to some old version. The ability to have a link always refer to the latest version is useful. The same applies to torrents. It's possible for the magnet link to the latest version to get lost and then a bunch of people are accidently downloading old, worse versions of the file with no way to find the newest version.