What Does Crest White Strips Exactly Do?

I just bought a box of crest strips but i’m not sure if it worth using it.
Do the strips just cover up the yellowness or do they actually clean the teeth? Because if all they do is cover up the yellowness then I would rather go manual brushing instead spending alot of money on strips. Also, if I do decide to use the strips, do I need to keep using it forever or can I just use a few and that’ll keep me good?