It also has no effect on whole classes of vulnerabilities which don't rely on unusual writes, where the system (SQL or LLM) is expected to execute some logic and yield a result, and the attacker wins by determining the outcome.

Using the SQL analogy, suppose this is intended:

    SELECT hash('$input') == secretfiles.hashed_access_code FROM secretfiles WHERE secretfiles.id = '$file_id';

And here the attacker supplying a malicious $input so that it becomes something else with a comment on the end:

    SELECT hash('') == hash('') -- ') == secretfiles.hashed_access_code FROM secretfiles WHERE secretfiles.id = '123';

Bad outcome, and no extra permissions required.