Does a Python node subscribed to CompressedImage messages need to use image_transport republish?

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I have a Python ROS node that subscribes to a topic of message type CompressedImage. So far it works fine (I play a rosbag with compressed images, run my node, and it correctly reads them [I checked using cv2.imshow()].

I'm probably misunderstanding what the point of compressed_image_transport is. My question is:

If I have a ROS node (say usb_cam) publishing compressed image messages, and my Python node subscribes to those compressed images (specifying sensor_msgs.msg.CompressedImage as the message type), why would I ever need image_transport or republish for my node (I know image_transport is being used in the publishing node, i.e. usb_cam, but not in my Python node). What's the point of image_transport republish in such a case?