Speaking in general terms, when working with enamel paints, a primer is not essential. Having said that, I do prefer to lay down a "primer" coat but it is also an enamel paint and not a special primer specifically for that purpose. This is true even when working with metal parts such as an aluminum gun barrel.
As Aces points out, using a primer can produce several benefits depending on what colors you are using beyond just the standard primer application.