I read somewhere that setting make-up with powder will make it last longer. I read about it working with foundation but I don't have any powder and barely wear foundation anyway, so I decided to test it out using pencil liner and black eyeshadow. The right side has the liner and powder and the left side just has pencil liner.
At the end of the day (and it was a lonnnng day) I took a picture of the results.
And they both looked about the same. I do think the right eye with the powder had more black on it. But I don't think it's worth the extra effort to add powder, I would just rather carry the pencil around and add more as it wears off.