Why do you need to light a lamp step by step in portrait photography?

This is relative. There are several kinds of lighting commonly used, which should be selected according to the scene and purpose. If you shoot in a studio, you don't need to shoot one by one, because the mode is fixed and the emphasis is on speed. The faster people get up, the better the background, and there is no need to look at the effects one by one.

Other aspects, especially scenery or unfamiliar with the environment itself, such as business or creative portraits, are best to come one by one, because with the help of environmental lighting, a photographer who relies on the effect of the first light to choose the position of the second light, or a photographer who doesn't need experience at this time, will quickly experiment and decide to start shooting around a beam of light and say nothing to a strange environment.