WinRAR cannot modify locked archives, so locking important archives prevents their accidental modification by WinRAR. Such protection might be especially useful in case of WinRAR commands processing archives in groups.
This switch is not intended or able to prevent modification by other tools or willful third party. It implements a safety measure only for accidental data change by WinRAR.
Example
Add srcfiles to final.rar and lock resulting archive
WinRAR a -k final.rar srcfiles