Why is it important to apply a primer prior to painting?

Because it will avoid problems with

• old oil-based paint (you ABSOLUTELY need a primer over an oil-based paint before painting with water-based/acrylic pain)
• patches made of different material (patches of plaster for example) that would react differently from other parts of the wall
• bare surfaces (new walls) which won't give good adherence
• colour changes (if painting over walls of different colours, the result will look different with the first layer(s) of paint. The primer can help)