The ability to append to a block already existed with block blobs, which are a bag of data blocks and an ordered list of block identifiers: you could just create a new block, then commit a new list with its identifier at the end.

The real benefit of the new append blob is that you have a one-request append (instead of read list, upload block, commit list).

Also, append blobs (like block blobs) are limited to 50000 append operations.