Launch detection was, I believe, not all that robust in the early days, and making sure that the enemy couldn't precisely target all of your silos, and making sure they knew they couldn't, was presumably useful for deterrence.
There's a good chance some counter launches get delayed... Seems like a lucky first strike could significantly reduce the counter strike. But everyone loses anyway.
And ICBMs these days are all solid fueled, I don't think they need much preparatory time.
And it’s why the SSBN is such a deadly second strike platform. If you know the nukes flying at you, you send the global alert, the SSBNs stationed secretly close to enemy coastlines surface to get emergency orders, and then let their nukes fly potentially before the first strike wave has had time to hit their targets… I’ve even seen some war game scenarios where the second strike SSBN based nukes can hit the enemy before the first strike reaches the original targets (obviously this is theoretical war game scenario stuff so it must be taken with several grains of salt, but it’s a surprising realisation that submarines can in the best case scenarios, put nukes on your enemies faster than their first wave of nukes will get to you… )