When is "no rest for the wicked" used?

I've stumbled over some dialogue in a textbook, where some people have a chat at work, and at the end, when they get back to their urgent tasks, one character says (emph. mine):

Well, I'd better let you carry on then. No rest for the wicked.

Now, I've found out where the phrase comes from, but I do not quite follow in which context it can be "used for mild comic effect".

Solution 1:

It's an idiom, generally used when somebody is expressing their belief that they (or someone else) has to work without rest. 'For the wicked' is a somewhat tongue-in-cheek expression implying that the reason the person is having to work is as a punishment for their being wicked, but this meaning is never literally intended.