Why is it so important to use a primer before wall decoration

The primer is a very important and necessary material in construction. In finishing work, for example, in plastering, it processes the walls and stretch ceilings of Yekaterinburg before the start of the work itself. Before applying each layer of plaster or putty, the primer is applied to the supporting surface of the walls and ceilings. And before painting or pasting the walls with wallpaper, they are also treated with a primer. The primer is a liquid, usually white. True, a primer and in the form of a thick pasta is produced. Such a putty is called concrete contact, and are used to process highly contaminated surfaces, with the remnants of paint or fat spots on them, as well as to prepare concrete surfaces for plastering.