The major significance of the McGuffey Readers is that they were a foundational piece of American schooling for more than 100 years. This allowed them to have a major impact on the socialization of American children.
The McGuffey Readers were meant to inculcate a certain set of values into American children. These were very conservative values that centered on religion and on patriotism. The fact that these readers were so widely used for so long helped to make these sorts of values cornerstones of American society. It is because of these value-laden aspects of the readers that they are still used by some private schools and homeschoolers.