> The real short of it is that it's a character that existed in another character set, and its role in that character set appears to be limited to "minor printing mark,"

Not really. The printing mark itself wasn’t a “character” in any character set until Unicode. It was probably a custom SVG-like shape that could be plopped into the margin of the document in desktop-publishing programs; sort of like drawing any other arbitrary vector shape (e.g. a 13-agon.) In specialized desktop-publishing programs (e.g. musical-score “engraving” programs), all sorts of these custom vector-shape symbols come with the system for placement onto the document; but—until very recently†—these weren’t “text” in any sense, just shapes.

† https://blog.dorico.com/2013/05/introducing-bravura-music-fo...

What was a character in a character set (ISO 5426-2) was a sort of “reference to said printing mark” or “exemplar of said printing mark”, used once in a standards document to give examples of how printing marks are used. Note the difference: the printing mark itself is an arbitrarily-complex and varied shape that different desktop-publishing programs might render in all sorts of ways. The “reference to the printing mark” is one mark that appears in the standards document, one way. (Picture the difference between an actual calligraphic flourish separating passages of text, and a Unicode fleuron/heart-bullet. One is not the proper encoding of the other! The former is a shape, not a character per se. The latter is a character: it’s a standard shape-thing with a standard meaning, that communicates meaning, and is considered “part of” the text.)

Unicode’s mission statement is essentially to offer an encoding for the set of all characters needed to losslessly re-encode all document-corpuses (corpii?) of historical importance, such that those corpuses may be re-encoded into Unicode for archival purposes, with the Unicode standard then acting as the record-keeper of the semantic meaning of the characters used in those documents. Without this work, over time we’d lose the semantic meanings of those encoded characters, as the original living systems that represented the encoding began to rot/disappear. We’d just be left with opaque bytes in the middle of a document that we’d have to guess at the meaning at.

The printing mark, as used for its intended purpose, doesn’t meet Unicode’s mission statement, because nobody’s encoding it into a (preserved) document, only “drawing” it on page-edges before trimming them off. It’s not “text” per se. It doesn’t appear in any electronic stream-of-text in need of archival preservation. It exists only in the same way an image embedded in a Word document exists: as self-contained data interpretable by the document-publishing system, an “add-on” on top of the text, but without the meaning of the text depending on the image. The document-publishing system “brings together” a text source, and other data, to create a final “layout” that is more than just text per se. The text source, not the final layout, is what Unicode is concerned about.

But the “exemplar of the printing mark” ended up in a standard about document binding. And that glyph is “text”, in the same sense that an emoji is text — i.e. it’s part of the corpus that Unicode seeks to preserve, in order to preserve the meaning of the text surrounding it. So it must treat the mark there as a character.

(Yes, this means that if a text of historical importance has some odd squiggle in the middle of it — and it then talks about the squiggle — that squiggle will inevitably become part of Unicode. That doesn’t imply that a visual for the glyph will be drawn by every font artist, though! Just that Unicode will retain a codepoint defining it in the standard, to document the meaning of the glyph as it appears in that one archived document. It’ll likely appear as a replacement-box character, but people can go look up in the standard what that particular replacement-box character means, and that’s all a historian needs.)