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
Primary: Saison, Berliner Weisse, sLambic I, sLambic II, IPA, 70/-
Secondary: Orange Blossom Mead, Flanders Red I
Kegged: Fat Man Porter w/ 1469, Fat Man Porter w/ 005, Centennial Falcon w/ Conan, Centennial Falcon w/ Denny's, Barrel-aged Fat Man Porter, Belgian Dark Strong, Dark Mild