This syntax has been adapted as a variation of the <URL:[...]> convention
from RFC 1738 (refer to its Appendix).
The form appearing here is a more general derivation, which (a) allows the
omission of the "role" atom† ("URL" in RFC 1738), with clients instructed to
intepret the omission to mean that "URL" is implied; (b) better accommodates
more recent conventions that omits both this atom and the colon separator
entirely, encoding instead just the URL, as in <https://example.com>; (c)
doesn't lead to interference with the HTML5 parsing algorithm. The latter is
crucial for compatibility with mainstream browsers--the leading colon prevents
the the bracketed plain text from being interpreted as HTML.
Maybe if we were to spec this out, we'd allow an alternative for plain links
of the form <@https://example.com/> (which permits no "role"--and is
therefore only used where "URL" can be inferred)?
We also want there to be a way to denote links not meant for direct
navigation, but rather things that you would otherwise use the Link HTTP
header or the link HTML element for. Some candidates:
† NB: '"role" atom" is terminology of my own choosing. The RFC's authors only
ever refer to it as a "prefix".