Why do you need to bring a gray card or gray paper with you when shooting outdoors with a SLR? What is its function, and how to use it in actual operations?

Adjust the white balance? I guess it's to adjust the white balance. The purpose of adjusting the white balance is to ensure that the photos taken will not have particularly large color differences. It looks like: Via specific buttons or menu items, your camera provides controls that allow you to adjust the white balance setting to match the actual lighting conditions.

Generally speaking, cameras have this option. When you face this piece of paper, the machine will automatically adjust it. This is automatically adjusted. It seems that it can also be adjusted manually