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...