In-universe, he's the first mutant of the modern age, the first mutant in the mutant baby-boom. (Technically he's not, but let's pretend.) And you could make a pretty good argument that he's the first Big Name Mutant, which is definitely the case: the Marvelverse 1930's had quite a few superheroes, but by this point, all anyone remembers is Namor and Torch, and Torch is a robot.