system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the point of countries having nukes if they can cause so much damage, will likely never be launched, and can just be shot down?
gpt
They can't just be shot down. 

Many of them are MIRVs, which means that their warheads are actually a bunch of smaller, independently targeted warheads crammed into one shell. A MIRV can have as many as 24 warheads, all capable of decimating a city, and it may also release chaff that can confuse anti-warhead targeting systems. If you can launch just 20 of these, the target would need potentially up to 480 ASATs to stop them, and most countries with nuclear weapons have many more than that. Not many countries even have 480 ASATs to be begin with, much less the thousands needed to stop every single warhead. 

Short-range missiles have a good chance of getting past defense systems, especially if shot very close to the target. The Cuban Missile Crisis happened partly because it would give the USSR the ability to launch missiles that could strike within 20 minutes, which is barely enough time to get into action stations and *start* priming missile defense systems. 

The idea is the same as having any kind of weapon. You have it in case you need it, because it's better to have it and not need it than to need it and not have it.