why should one use the harvest media?

It is recommended to use Harvest media that contains more FBS than Seeding media to provide the 293T packaging cells with additional nutrients while producing the virus particles. Also antibiotics (Pen/Strep) are added to the Harvest media to avoid bacterial infection of packaging cells. It is not recommended to use antibiotics in Seeding media while transfecting the cells using Lipofectamine 2000 as this causes cell death.