Users sending a spoiler could use this command to show it only to the people that really wants to see it. When using /spoiler followed by a message the chat will instead display a "Spoiler" button which when clicked will show the message.
/spoiler The cake is a lie!
/spoiler The girl dies at the end of the game!
/spoiler The princess is not on this castle!
Not really sure when this would be useful.
Sounds like it would be abused by trolls and not very useful as chat is always moving up.