This was used to streamline conversion between ints and gmp-based integers. E.g. in the gmpy2: https://github.com/aleaxit/gmpy/blob/e0944795008e6929c3d6cdfabb8344ce8a9e99d6/src/gmpy2_convert_gmp.c#L148-L181 or in the Sage: https://github.com/sagemath/sage/blob/2f1a76dc24af6c8ca9a804b10913c33a936b4fb8/src/sage/libs/gmp/pylong.pyx#L57-L69

Clearly, we want a better C-API e.g. for import/export ints, like mpz_import/export (probably, #102471 is the right metaissue here), but I don't see good reasons to hide this function now, without any alternatives.

See #106320 and #108604.