> [1] and who's to say there aren't multiple signed hashes on the blockchain anyway?

The timestamping of the blockchain is the use case. If multiple signed hashes exist, the latest one supersedes all the previous.

> What do you gain if the lawyer has to reveal it anyway? If you don't trust the lawyer[1], just give a hash of the document to the people who would be affected.

I'm afraid I don't understand. Who would hold the original text of the document in your plan?

Edit: Nevermind, I see what you mean, here. I see the issue as one of public record. If there was a revision to the will which no one in the family liked, and benefited some estranged cousin of yours who lived in another country, the family and the lawyer could conceivably hide the new will and the fact that you were dead from this cousin. Having it all on the blockchain prevents this.

The blockchain places the production of the hashes in time; in this case showing that - all else being equal - this will supersedes any previous wills.

Proving things existed at a particular date is important for contracts, wills, IPR, and such.

It doesn't solve everything, you could have a collusion between relatives and lawyers such that they created multiple blockchain entries. But the blockchain would show, for example, if they did it after the subject had died. The relatives would also need to buy off those running the public blockchain, and not just the deceased's law firm, in order to get a legal attestation -- and audits would show the public servants lied. Auditors can be bought off too, no system involving people is perfect.