Why Kanban cards let downstream demand control the line — instead of a forecast that's guessing at it.
Every production stage has to decide when to make the next unit. There are only two fundamentally different ways to answer that question. A push system answers it with a forecast: make units on a schedule, then send them forward whether or not the next stage is ready. A pull system answers it with a signal: make a unit only when the downstream stage has actually used one and asks for a replacement. That single difference — schedule-driven vs. signal-driven — is the entire reason Kanban and Just-In-Time production exist, and it has direct, measurable consequences for how much work-in-process inventory piles up between stations.