How can I trust a particular calendar?
You trust different websites in different ways. The proof of ownership is the domain name ("example.com"). You might have a different relationship to theguardian.com than 4chan.org, for example.
By making a link in a standardardized manner from your website to a calendar, you prove that you "endorse" or "stand behind" a particular calendar. It doesn't mean that you own and control the calendar, since a website can link to someone elses calendar, but it adds trust.
Since the link format is standardized, we can easily automate the discovery process.
Let's say The Guardian publishes a public calendar of newsworthy world-events. Maybe they publish this calendar via the web page theguardian.com/events. If you visit this url in a browser, you get a regular web page that explains the purpose. However, I can add this calendar directly to publicdata.events, without knowing the actual (and complicated) url to their public calendar.
In other words, theguardian.com/events is how the calendar is identified and referred to. By calling it like that, you automatically know you can trust it in a similar manner as theguardian.com.