As TS says, in a balanced wort it would be a waste . . . however even an all grain wort can be deficient in zinc which is required by yeast in microgram quantities, so a tsp of yeast nutrient can help . . . it will also help in kit n kilo brewing as it boost the 'low' Free Amino Nitrogen which is also required for good yeast growth . . and fermentation.
In short, it will do no harm to add a couple of tsp of it to your beer wort, and may very well do some good . . . but in an all grain beer it is not a requirement