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 compressedimagetransport 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 imagetransport or republish for my node (I know imagetransport is being used in the publishing node, i.e. usbcam, but not in my Python node). What's the point of imagetransport republish in such a case?