They never explained in the games why either of them exist. Jasmin in the original pair called Steel "Newly discovered" but there was never any mentioning of Dark Type. They just sorta always been there. In the remakes, Jasmin's little bit about steel was removed. That was the only reference of a type being discovered.
Out side of the games, they were added to tone down Psychic and the Dragons coming in. Steel brought some heavy balancing, but Gen 2 did way more than just add new types, the type chart got a make over.
Bug's weakness to Poison was removed, and likewise Poison's weakness to Bug was removed.
Poison resisted Bug now.
Psychic was made weak to Ghost instead of Immune.
Fire was made to resist Ice.
5 moves may seem miniscule but they are what set our current Meta to what it turned out to be along with specific Pokemon having fairly abusive stats.
There is a chance they may do another Type chart shuffle with the new type addition just to further change the current Meta.