The foundation is what provides a good portion of the stability for a structure. When there are issues with the foundation, the entire structure might be unstable and unusable. This isn't ever something that you want to deal with in a new construction.
When you hear the term construction defects, you might think only of something being built incorrectly. While it is true that this is one type of defect, there are actually many types of defects that might occur in a building project.