It’s 11:59 PM on Black Friday. Your e-commerce platform has been running smoothly all day. Then at midnight, 500,000 shoppers simultaneously hammer your /checkout API. Your servers start queuing requests. Then they start dropping requests. Then they crash. Every second of downtime costs thousands of dollars in lost sales. Meanwhile, a competitor’s site — running the exact same infrastructure — handles the load just fine. The difference? They had a rate limiter.