While primer is not always a requirement, it is always recommended, especially if the metal will come into any contact with moisture. Untreated metal requires primer to seal it from exposure before any topical paint may be applied. Without the use of primer, in many cases, oxidation will begin leading to rust and the eventual decay of the metal. In addition, the chemical makeup of paint is less binding than primer meaning it will flake off easier without a primer underneath it.
How Primer works on Metal
Painting alone oftentimes does not provide a permanent coating. This is especially true for metal. Paint easily flakes off of metal if applied directly to the surface. To prevent this, primer is used. It is like paint except it is designed to fill in the microscopic gaps in the material and stick to it. Applied over primer, paint adheres much better, for it easily bonds with the primer.
Primer also works as a protective agent. It resists moisture for materials that have no inherent water resistance. With most metals, without primer, not only would the paint flake, but oxidation would immediately begin once exposed to moisture.