Significance of guidelines, explanatory notes and other "soft law" acts

In the European governance the EU legislation distinguishes between the so called “formal law” and the “informal law”. The latter also known as measures of soft law. Formal law compromises regulations, directives and decisions, whereby recommendations and opinions, as well as other methods of developing policy: e.g. policy guidelines are considered soft law. Practical examples of informal law in the field of customs are the Guidelines for Customs Valuation of Goods, the Customs Valuation Compendium, the Transit Guide. What is the significance of these documents in practice?