What are some use cases of changing the fixed frame in rviz ?

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You may want to change the fixed frame so you can view something in another frame of reference. It can be useful when debugging information, transformations, or wanting a steady look. There are as many reasons to want to view data in a certain frame as there are frames to look at the data in.

For your odom example, you may want to do this so you could view data in a frame unaffected by global localization updates and is promised to be smooth and continuous for measuring drift in odometry using sensor measurements to determine ground truth. Or perhaps I'm interested in looking at a robot's sensor data while navigation isn't running, so visualizing it in the base_link frame will work regardless of the map or odom transformations are available.