Complete the job, as directed. There is a comma. why?

The comma after “job” tells us that the phrase as directed is non-restrictive.

The sentence states “you have been directed to do a job”, and implies that how you do it is up to you.

But if we take out the comma,

Complete the job as directed.

Now “as directed” is restrictive, and the sentence is saying something more severe: Do the work, and make sure you do it the way you were told to do it.

As Dan has said in his comment, the comma adds gravitas. However, I believe it also changes the implication of the sentence.

Complete the job, as directed

could be interpreted as "You have been told to finish this task. Do so.", which says nothing about how you should perform it. In contrast, I feel the clear implication of

Complete the job as directed

is "Finish this task in the manner which has been specified".

So it may be that adding gravitas risks reducing the actual authority, in this particular case.

With a comma: focus is on job completion and as previously suggested says nothing about how it is to be completed. The alternative without a comma has the sense of an imperative command that says do the job as directed in the way directed, but the punctuation is incomplete. With out a "!" ending the sentence the meaning and expression is weak and somewhat vague: as though spoken by an a person lacking confidence. Try: "Complete the job, as directed!"