The use of cytotoxic T-cells
I didn't watch the video, but the answer you're probably looking for is that cytotoxic T cells are for intracellular infections -- that is, CTL target pathogens that grow inside one's own cells.  All viruses grow inside cells. Some bacteria also grow inside cells (e.g. tuberculosis, Listeria); those bacteria can (often) be effectively attacked by CTL. Many bacteria do not replicate inside cells, and for those, CTL aren't effective, but B cells (via antibody) and T helper cells (by recruiting other cell types and modifying inflammation) can still be effective.