The other comment is correct, it was added as part of proposal adding a larger set of mathematical symbols[0]. The wikipedia page actually mentions the path through which it was added, which lets us make some educated guesses:
> From that apparent beginning, the Angzarr was swept up into the Monotype typeset catalog of arrow characters (...) It is unknown why Monotype added the character, or what purpose it was intended to serve
> In 1988, the International Organization for Standardization added the symbol to its Standard Generalized Markup Language (SGML) definition, apparently pulling it from the Monotype character set.
> In March 2000, the Angzarr symbol reached wide distribution when the Unicode Technical Committee, in collaboration with the STIX project, proposed adding it to ISO/IEC 10646, the ISO standard with which the Unicode Standard is synchronised. The Angzarr was proposed in the ISO working-group document Proposal for Encoding Additional Mathematical Symbols, although no specific purpose is listed for the symbol.
My guess is that the people proposing the addition of new maths symbols[1] weren't going to decide on inclusion or exclusion of a symbol on the basis of being familiar with it themselves or not, since that was likely true for many symbols that happened to only be used in fields of mathematics that they were not working in. Meaning they had to rely on some other kind of "authority" to infer that a symbol was used by the larger maths community. With that in mind "being part of the Monotype catalog and part of SGML" seems like a pretty sensible heuristic to go by.
Another consideration might have been that they simply wished to have complete coverage of the symbols that SGML encoded, regardless of familiarity with the symbols involved. And of course both could have been true.
[0] https://www.unicode.org/wg2/docs/n2191.pdf
[1] https://en.wikipedia.org/wiki/Angzarr