Nuclear deterrence - why do we need missiles?

I recently visited the Titan Missile museum in Arizona. Fascinating. If I understood correctly, detonation of about 20 of those missiles could have plunged the world into a nuclear winter eliminating all life on earth. My understanding is they had 54 of these missiles across the US and if fired upon they intended to launch 54 in retaliation. So end of world. If I understood the above correctly, I’m curious why they felt the need to deploy these nukes via missiles? Wouldn’t a local detonation of 54 nukes have the same end result and the missile was an unnecessary complication? I suspect I misunderstood something so I’m hoping someone can explain.