For example, which is the difference between these:
The other answers list some superficial differences, but really miss the mark of the key difference that explains why browsers/spec writers would essentially duplicate something that already exists:
<- Secure in modern browsers, insecure in legacy browsers with no sandbox support
<- Secure in modern browsers, secure (though non-functional) in legacy browsers
This new syntax provides content authors a way to protect their users, even when they may be using legacy browsers. Without it, content authors would be reluctant to use the sandbox feature at all, and it would not see use.