Many S7 instructions configured in certain PLC's (like an output coil in an S7-316 rung), require an input element to precede an output instruction. If I wanted to force an output on all of the time in these PLC's, I would need to use the ALWAYS ON bit.
SAVE and SET are mostly used for program diagnostics and troubleshooting. SAVE saves the Result of Logic state in the BR bit of the executing block status word. SET sets the BR bit of the executing block status word. Then by querying the BR bit, external logic can be executed when an error is detected.