Why do some documentaries have to film a person's walking steps first?

Yes, that's not true. You don't have to walk a few steps first. First of all, walking a few steps is just one of the photography techniques, which is a montage shooting technique. You can also use a long lens to highlight the vastness of the scene, and walking a few steps is only used for the appearance of mysterious figures. Ask me if you have any questions. . . Thank you.