JSON has special characters: quotes, backslashes, newlines. To include these in a string, you need to "escape" them. That means turning a newline into \n, a quote into \", and so on.
When This Matters
If you're working with APIs, you'll hit escaping issues constantly. Your JSON validates fine on its own, but inside another JSON string? Broken! A good escape/unescape tool fixes this instantly.
This is one of those things that seems obscure until you're debugging an API call at 2am, and then it's your best friend.