There are seven good reasons to paint or powder coat hot-dip galvanized steel:

1. Extended Durability in what is called the synergistic effect between galvanized steel and paint, the zinc of the galvanized coating is a barrier protection to the substrate steel, cathodically protects it, and provides underfilm corrosion protection of the paint.  Synergistically, the paint provides a barrier protection to the zinc coating.
2. Economic because the zinc extends the life of the paint, less maintenance is required over the entire service life.
3. Ease of Repainting when the paint of a duplex system completely weathers, the exposed zinc coating requires minimal surface preparation when repainting.
4. Aesthetics   desiring a color to match surrounding structure or for branding.
5. Safety Marking the extremely high cost to maintain a very tall structure often means galvanizing is a requirement but those same tall structures are mandated to be painted safety orange and white for high visibility to air traffic.
6. Extending the Life of Previously Galvanized Steel large in-place structures often cannot be regalvanized after decades of exposure and so a logical means of keeping the structure in service is to apply a paint, often an inorganic zinc-rich paint which provides barrier protection and some cathodic protection.
7. Color Coding steam, water, and other fluid piping systems are often painting in industrial facilities to distinguish them for quick repair and safety concerns.

See Also:
Guide to Preparing Hot-Dip Galvanized Steel for Paint
Duplex Systems