It's great as a public immutable timestamping service. Make any transaction on a popular blockchain with a low transaction cost, and add a hash into the transaction. Now you can prove beyond the shadow of a doubt that you knew this hash at that point in time (precise to a couple minutes), and by extension if you produce content that matches that hash it proves that you had that content at that point in time.

You can use that for copyright or trademark matters without revealing the work itself, or you can ensure the integrity of your logs this way. No third parties that have to be trusted or that could be bribed.

Of course that doesn't really work as the primary purpose of a blockchain, to get good trust guarantees you need to piggy-back on a blockchain that does something more valuable, so it's not really a killer problem.