Navigation without local costmap?

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I'm using rtabmap to generate the global cost map. I understand that local cost maps are in the Odom frame instead of map and that it's for short term path planning. The local cost map takes the global map and sensor reading to create the new map.

But what I'm confused about is why I couldn't just use the global cost map as is? It already takes into account the sensors plus the rgdb camera. Is it because the global mapper can't run at high frequency?