Why do many models have to hold a black, white and gray card in their hands before portrait photography?

The purpose of the three-color card is to determine the white balance in the future.

This kind of shooting usually adopts RAW format, and three-color cards can be used to locate black field, white field and gray field respectively. When these three are confirmed, the color will be accurate naturally. The gray card part can determine the exposure more accurately. In fact, there are many adjustment methods for later RAW photography. At this time, there is not much need to rely on the gray card to determine the exposure.