Why do it, what happens if you don’t do it?
It is important your garage door installer does it to comply with EU legislation. For example, it is against the law to install a power operated door in the UK without complying with the EU Machinery Directive.
It is illegal as a manufacturer, importer or reseller to supply an electric operator within the EU unless it is CE marked. A garage door also requires a CE mark under the Construction Products Regulations. Once you have a garage door and an operator, each carrying their respective CE marks, as an individual you can fit them yourself and you don’t need to do any more paperwork. However if, as part of your business, you are being paid to fit a door with an operator (or an operator to an existing door) then you also need to complete a “declaration of conformity” under the EU Machinery Directive.