The risk is that some readers may be offended with the simplicity of the subjects covered. To those, I apologize. To the rest, let's learn some video basics. Sync Television uses sync for much the same reason that film uses sprocket holes. The only way to control either is to use a known reference, like time, and something attached to the particular picture.
Film only has plain old sprocket holes.
Television has many types of sync signals, with different purposes. Coordination between cameras and display monitors is accomplished by vertical and horizontal sync. These two are almost always combined into what is known as composite sync. This combined pulse, when added to video, is that which turns video into composite video. Composite sync is used often in current video systems.