Clayton,
In previous releases of MATLAB, there was only the Robotics System Toolbox, which had some ROS and navigation applications. In R2019b, this toolbox was split into three, and each toolbox was augmented with more focused tools for their applicable area. All three toolboxes are still useful for robotics in general, but an individual may not need all three for their focus area of robotics. Also, navigation and ROS communication is also applicable to areas outside of robotics (like automated driving), and so can now be used separately of the Robotics System Toolbox.
There is some similarity between them (like using the same transformation and quaternion conventions), and some cross-applicability (like the Navigation Toolbox's SLAM Map Builder app importing from rosbags), but that tends to be true across many MATLAB toolboxes.
-Cam
