NHSAA offers accreditation to homeschool families in exchange for a fee. While they provide some services like a transcript service, the accreditation itself doesn't necessarily mean a higher standard of education than a homeschool that chooses not to be accredited through them or another organization.
In short, NHSAA is a for-profit company offering a service to homeschoolers who desire accreditation, but its accreditation doesn't hold the same weight as accreditation from state-recognized agencies (where such agencies exist) or regional accrediting bodies for traditional schools. The value of NHSAA accreditation is largely dependent on the individual family's needs and goals and how colleges or other institutions view it.