The main use case is privacy... But theoretically you can program anything so it also privately extends script... Of course the problem is that this only works while the signer is available, if it isn't you need to have a fallback with CSV to something else.
And if the signer isn't actually blind then they might be able to steal...
So it is great to have, but we still need covenants so you can have delay functions to watch a stealing attempt and react to it in time with a watchtower.
