It is important to understand that one product cannot perform all the functions required of a wall coating
In a paint system, there are a number of different layers. e.g. metal primer helps to prevent corrosion, undercoat to provide build and filling, and a glossy topcoat. One product cannot perform all these functions. Because of numerous methods of application such as brush, spray, airless spray, and dip, it is not possible to formulate one paint to suit all these requirements, as different viscosities are involved.
For most retail and trade markets, paints are expected to dry under normal ambient temperatures. Industrial customers however, due to speed in production, often require paints which dry extremely quickly, or have to be heat cured. Again, it is not possible to have one paint to meet the many different drying times that customers require.
Paints are also available at various gloss levels, ie matt, sheen, semi-gloss, high gloss etc and these, like colour, are used for aesthetic reasons.
Customers are considerably influenced by the cost of paint, and as a result paint manufacturers have been forced to supply products in various qualities at different cost levels.