The helps the primer act as a link between the moist tooth surface and the bonding resin. The question is, does the primer do the same thing for enamel that it does for dentin. This is a question with very practical applications because commonly in bonding situations, the enamel and dentin surfaces are juxtaposed. This makes it almost impossible to keep from applying the primer to enamel, even if you don't intend to. Unfortunately, research has been somewhat equivocal on this question.
Some studies show that applying the primer to etched enamel reduces bond strength.
It is important to note that in all these studies, the primer was applied to etched, dried enamel. Recently, a study was published that evaluated the effect of three dentin bonding primers on the bond strength of resin composite to moist and to dry etched enamel. After measuring the bond strength to various combinations of groups, the results were that for the multi-component products, the bond strength was unaffected when primer was applied to etched, dried enamel. The bond strength was significantly increased when primer was applied to etched, moist enamel. For the one-component product Single Bond, moisture on etched enamel did not affect bond strength.