Medieval scribes marked the start of a new argument with a rubricated pilcrow, but rubricators were a separate, slower step added after the main text was copied; scribes left gaps for these marks that rubricators often never filled in. The empty indentations hardened into the paragraph indent we still use, so the modern paragraph break is literally the negative space left by a symbol that was never drawn.
The ampersand began as a handwritten fusion of the Latin 'et' (and), the two letters run together so often by Roman scribes that they merged into a single glyph. A frequently repeated sequence can abbreviate itself through the physical economy of writing, eventually becoming an independent character divorced from its source letters.
A mark's survival depends less on its usefulness than on whether successive writing technologies can reproduce it: the manicule and pilcrow faded as movable type made them costly, while the octothorpe and @ sign were rescued from obscurity by telephone keypads and email addressing. The medium's constraints and inclusions, not inherent merit, determine which symbols persist.
The interrobang was deliberately invented in 1962 by ad executive Martin Speckter to combine question and exclamation for rhetorical queries, given a name and even cast in some typefaces, yet it never achieved lasting adoption. Symbols coined top-down by design, unlike those that accrete from use, tend not to take hold even when they fill a genuine expressive gap.