Content-addressed URL. This allows you to link to medium-sized texts without:

• having to create a page first, or
• the data being lost if the site goes down.

Your text:

Only plain text is allowed so that I hopefully don't have to police this. The size limit is a few KB—systems don't like URLs beyond a certain size anyway.

An alternative to this system, is using data:text/plain,... (data URL), but this is often not parsed/clickable, or not allowed altogether.

One of the possible uses is making footnotes, e.g. the link in "clouds can weigh a million kg[url=https://...]¹[/url]" or "clouds can weigh a million kg[¹](https://...)" could be a content-addressed URL with more context (like how you calculated the value, or conversions to other unit systems).