Powder coatings aren't applied in the typical paint method. That is, the paint isn't in liquid form, and it isn't thinned or reduced with another catalyst agent that will evaporate.
Powder coating generally involves an electrostatic system of applying powder to the item, which is then heated to 400°F to allow the coat of powder (hence the name) to fuse together.
Due to the heat required to "cure" the material, this is already a "hard" paint. Additionally, you know that it will be literally FUSED to the piece. Also, the amount of time to get this finish is relatively quick as compared to the older spray system.
The biggest advantage over sprayed coatings is that since it has literally been melted onto your piece, there are usually no seams or gaps in the coverage. Additionally since there are no solvents within that need to evaporate, the porousness of the coating is literally nill. This makes for a very smooth and impervious finish.