If you don't add it, in many cases (mine, as an example), the carboy will develop a milky ring on top of the wine. As you rack the wine, the ring will run down the sides and coat the whole insides of the carboy.
If you let the racked wine set for a couple of weeks in the new carboy, the ring will form again.
The enzyme packet, when added exactly when the instructions say, will eliminate this.
I have never made a kit but first why would you leave it out? If it's pectic enzyme it will assist the yeast to break down the sugars in the juice and help prevent a haze in your wine.
Do not add it with any sulfite. Allow 12 hours between the two.
I don't know if it is pectic enzyme or not. It is just labeled as "Enzyme". This enzyme pack only comes with kits that have a grape skin pack. I did not intend to leave it out.