Why is a lot of RAM required for a minecraft server?

From what I understand, RAM is used mainly for doing multiple tasks at once, if your not doing a lot of things at once then you don't use much RAM, so if all I'm doing is running a server on the computer, why do i need a lot of RAM?

Well, for starters you have multiple people connecting concurrently to your server doing things, so you have at least 2-4 people doing things all at the same time in their own clients with the world being updated in real-time on the server.