Many of the programs developed for schools are not intended to replace teachers but instead are designed--at least originally--to supplement, or add to whatever teacher-led curriculum is taught.
Supplemental materials may be delivered online or may be provided in the classroom by a teacher--they're simply outside the scope of the official curriculum.
It may not be easy to "link" supplemental material to the teacher-led lessons. The easiest material to link directly to the lessons taught in class are "assignable" programs. If a teacher has structured a class according to, say, either state or Common Core standards, then it may be possible to "synch" supplemental material according to the standards.
Alternatively, teachers may let a program set the pace for learning; the teacher then can choose to sequence other materials and classroom discussion accordingly.
Frequently, such supplemental computer-based instruction offers opportunities to either reteach concepts or drill on concepts along a track that may be parallel to teacher-led programs.
Note that "supplemental" materials are different from "specialized core" materials, which are curriculum on subjects other than math, literacy, science and social studies. (Economics, for instance, can be a "specialized core" subject.)