Yeast nutrients tend to be in a more labile form than in wort, so the yeast can utilize them more quickly and efficiently. Also, wort can be deficient in zinc, which yeast need. You'll be fine without it, but if used as directed, you may observe shorter lag times.
The Fiesty(sic) Goat Brewery est. 2007 & Clusterfuggle Experimental Ales est. 2009
Planned: sLambic II, Flanders Red II
Primary: Fat Man Porter, Orange Blossom Mead, sLambic I
Secondary: Flanders Red I
Kegged: 3 different ciders, Janet's Brown, Barrel-aged Stout, Hallertau Blanc Pale Ale
Bottled: Flanders Red I