I guess Rajzor wanted to say that for an virus to activate on your computer, it has to be started. It cannot be started from an archive - it has to be unpacked first. So, if you have the antivirus resident protection running, it will detect the virus and prevent its execution at the moment you try to execute it (after it’s unpacked) - even if the detection inside the original archive fails.
The scanning of archives is just another layer of security.