system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the point of SLBMs if we already have ICBMs?
gpt
Submarine-based missiles have a few major advantages.

1. They can be launched very close to their destination. A missile hitting Pyong Yang from the Sea of Japan takes a lot less time than one fired from Guam, simply due to range. This gives the enemy a lot less time to detect and retaliate.
2. Since they are mobile and hidden, they cannot effectively be targeted by your enemies. In theory, an enemy could knock out any number of land-based missile launchers, since they are hard to hide and cannot move. 
3. They guarantee the ability to retaliate. This is part of the mutually-assured-destruction nuclear deterrent. If you strike an enemy and take them out with no chance of retaliation, then there's very little stopping you from doing it. If they have a sub that could launch a retaliatory strike afterwards, you have to weigh that risk.