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Because sensor's data has latency. If the latency is too large, the information you get from sensor will not immediate enough. For example, if sensor message's frequency is 1hz, and your robot's velocity is 0.5 m/s. It means that when costmap receives a data comes from the sensor, it's a second ago. And your robot moves 0.5 meter away from that point. The information provided by costmap will not be immediate enough, it may let your global/local planner make a wrong decision.