letlambda's explanation for IPFS is right. In Dat, that method is used to produce an append only log out of a merkle tree. Each time you add an entry, a leaf is added to the tree, and the new head is signed. I wrote about it here [1] and have some gifs demonstrating the process.

Merkle trees can work just fine for dynamic content. You just need to use a pubkey pointer.

1. https://beakerbrowser.com/2017/06/19/cryptographically-secur...

They certainly don't make that easier. But if the system supports a mutable link, you can have documents that always link to their own latest version.

IPFS does this by storing (mutable link, contentID) pairs in the DHT, which can be updated if you know the private key for the mutable link.