What does "Which is more than I can say for you." mean? When do you say it?

Question

What does  "Which is more than I can say for you."  mean?    When do you say it?

Answer

You say this to some one who has not met a standard compared to another person.

"Johnson has never missed a day of work before, which is more than I can say for you Smith."

Answer

It means that what was just stated does not apply (probably in a rather obvious manner) to the person you're talking to.

I like Yalqun's example sentence a lot.  I would add that in that sentence, the phrase makes it clear that Smith has indeed missed days of work and probably not just a few days: it suggests that he has missed a lot of them.