Basically, some type designers liked to play with capital ß and thought it cool to have it included into Unicode. There was a whole campaign for inclusion, and it was a big mistake.
Because even though existing use must be shown to merit inclusion, they only managed to find a single book (a dictionary) printed in more than a few copies. From East Germany. And that book only used the capital ß for a single edition (out of dozens of editions) and reverted straight back to double s. Somehow, that still was enough for Unicode.
Capital ß is a shit show, and it only confuses native speakers, because close to none of them have ever seen that glyph before.
It has no real historic lineage (like the long s, for example, that pretty much nobody under 80 knows, either), it is a faux-retro modern design, an idle plaything.