Why is The Twin Snakes a requirement in the Master Collection for so many people?

It's a genuine question, I never played the og MGS and only played TTS but from what I got, it's basically the same games except done on MGS2 engine with the upgrades coming along. Since Vol.1 had the original, why would they even port the GameCube game in the future if it's to give 2 times the same story?