If you are going to submit a job, a resume, or a document you don’t want changes to be made on, lock or protect PDF will let you protect the file with a password in a secure way. Thus, the original file cannot be edited.
Of course, there will always be some way to duplicate the file to be able to modify it, through something as common as unlocking the PDF. After all, there may be times when you mandatorily need to edit the document.