Primers or undercoat is used to prepare a surface before painting. Primers increase the durability of paint, provides additional protection for the surface being painted. As well as ensuring adhesiveness of paint.
When painting a wall it is important to use primer in order to use less paint. Primer seals a wall before painting so that less paint gets soaked into the wall. Priming a wall before painting allows the true colour of paint to shine through.
When skipping primer before painting, you are risking using more paint. And paint peeling, especially in humid conditions.
In the long run, it is a lot smarter to spend the money and time to use a primer before painting any surface to ensure that you have a quality end product.