[adrian_b]

The article mentions the fact that a successful second preimage attack is not always necessary.

A successful second preimage attack is needed if you want to make a second variant with the same hash like an already existing legal document.

However, when the original document is not yet in the possession of others (or there might be a way to destroy or replace their older copies), you can make more or less invisible modifications to it, so that a second different document will have the same hash with it. Then the altered original document can be handed to other parties, who will not notice changes from whatever had been agreed, while keeping an alternative document that can be shown later as having the same hash.

While opportunities for such a forgery should happen less often, it is much better to use a collision-resistant hash to completely remove this possibility.