A major benefit of append-only is that your writes are always ideal for whatever storage medium. Especially magnetic or tape. Combine append-only with batching of transactions (i.e. across 1-10 milliseconds at a time), and you can write multiple txns per disk I/O operation (assuming txn size < storage block size).
You can search for some comments I made recently about an append-only database scheme.
> everything would be append-only by default
doesn't mean everything is permanently etched into stone or written on the blockchain, it just means that "by default" everything you write is written to a new block[1], instead of having to free up old blocks to reuse and keep track of which blocks of storage are available
[1]"block" is just meant as a generic unit of storage, I'm not trying to say anything about actual drive blocks and implementation details
Do you know why ASCII 0x7f is DEL? Paper tape is write-once. To indicate a deleted character, it was conventional to punch holes in all bit positions -- 0x7f on a 7-bit punch.