An icon provides:
Visual representation, which can speed or replace the necessity of reading the text label.
Fast distinctive representation, which accelerates by a high factor the iteration in a large number of objects to obtain the desired object; we see a lot faster than we read.
Theses two aspects of the icons perception by the human visual system becomes more effective with the establishment of visual habits which permits to associate more quickly an icon to an object, a state or an action.
Icons guidelines help the user to adopt theses visual habits by promoting consistency. Guidelines provides common representations or ideas the user could associate to specific concepts. Theses representations could be numerous: colors, particular graphic elements, graphic style, etc.