[derefr]

Mobile-phone OS vendors like Apple and Google invent cute little pictures for people to send to each-other in text messages (and, mostly coincidentally, type into other apps and post to the web.) Then they tell the Unicode Consortium that they've invented these little pictures, and they're planning on putting them in their software, and so the Unicode Consortium better assign them codepoints, because if they don't, they'll just encode those pictures with a proprietary encoding and send them anyway, like ICQ and MSN did back in 2001.

And then the Unicode Consortium does give them a codepoint — because the whole point of Unicode is that there shouldn't be any text on the Internet that can't have its meaning determined 50 years from now because it was encoded in some proprietary now-lost encoding.

This is how it was from the very beginning. The first emoji codepoints in Unicode were added in order to have a valid projection-mapping into Unicode of the "SJIS with emojis" text written on Japanese mobile phones that became common on the Japanese Internet — including documents like "cellphone novels" with high historical preservation value.