One of the main educational advantages of science fair is that it requires completing a larger project than can be easily included in the short periods provided in most schools. Although writing assignments that take a long time to complete and require multiple drafts are fairly common in US schools, large projects in the sciences (other than science fairs) are rare. Science fairs also provide a mechanism for students with intense interest in the sciences to be paired with mentors from nearby colleges and universities, so that they can get access to instruction and equipment that the local schools could not provide.
A related source of criticism is the tendency for an inordinate amount of parental contribution to the projects, especially of winning projects. In the desire to see their children win the competition, many parents direct the children to choose projects far above a secondary student's capacity for understanding. Therefore, the parent or a connection of the parent with scientific or technical expertise will direct the development and execution of the project. Not only does this minimize the educational value of the project for the student, but also provides an unfair advantage to students whose parents have the technical connections and financial resources to invest in these projects.
Often, prizes in science fairs do not go to the best science, but to technology that is currently fashionable (green technology or health-related projects, for example). Judges often over-compensate for the possibility of parental involvement and downgrade advanced students who do work beyond what most of their peers are capable of.