Pacing and structure is extremely important in screenwriting because I believe that these elements can make or break a story. Without a proper structure, a story is going to get very confusing or feel really weird watching and without good pacing, a story will either move too fast, too slow and will ultimately make the audience feel frustrated (also, since pacing is often crucial to character arcs and other important story elements, bad pacing can make things drag out too long or even have crucial details be missed/failed opportunities to expand upon engaging bits of lore).