In order to make a virus you have to at least know a little someting about programming and the operating system with the software that runs on it. Then you make it do things you know the owner of the computer don't want you to do.
Scanning virus only help to find the virus inside your computer disks, but do no other actions, virus is still in your computer. But Removing virus not only find it, but also remove the virus from your computer. Thus no such virus will remain in your computer after that.