Split lock washers do nothing?

Huh? They always work. They do 3 things: They prevent vibration loosening by acting as a tensioner, they act as a flat washer once torqued down, and they can bite into soft metal to prevent the nut from backing off as they dig in on the CCW rotation of a loosening nut. Thy should only be used where they are called out, usually vibration areas such as starters and accessory drive brackets, but never under a head bolt.