Who’s this useful for?

Anyone who wants witness discount “unlimited” data storage on bitcoin. Op-returns have a smaller stack limit (even when removing the 80-byte “datacarrier” limit, the maximum allowed for a bare script is 10k bytes) and pay full price per byte. Annex data is a v1 taproot script. There is no limit on the size of v1 taproot witness scripts; instead you can’t exceed the blocksize limit of 4MB. Annex data is witness data, so it pays 25% of the fees the same data in an op-return or bare script would.

Another nice thing about using annexes for data is that it directly commits data into the chain, without needing to do the commit and reveal two-transactions step that the Ordinals Inscription protocol uses. It’s very similar to OP-RETURNs except it’s lower cost, allows for data up to the blocksize limit, and currently is not allowed to be relayed.