Two coats are better than one… but why? I mean, isn’t the purpose of paint to turn a surface a certain color? While most would consider the color to be the most important part, color is not the main function of paint(unless you’re Picasso). Paint is ultimately a functional waterproofing agent and sealant. It is a protective membrane. A membrane that performs best when it reaches the dried thickness(measured in mils) that it is specifically designed to have. Reaching the optimum specified thickness with one coat is not possible with most paints. This is functionally why two coats are better.
However, if we want to address the color issue, there are multiple reasons why two coats make the color better. Two coats provide a uniform and much more even color. There is also the issue of “sheen”, which two coats solves by building on the evenness and uniformity of the shine in the paint. This gives the surface greater wipe-ability and helps it to last longer.
So to recap, two coats of paint are better because they protect better, look better, and make your painted surfaces last longer.