When should Gazebo and ROS be used together

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It looks to me that your question is more about why you'd want to use ROS at all. The best way to have an answer for this question is to go through some of the ROS documentation and examples. Here's a specific page answering the question Is ROS For Me?

The two main reasons I can think of why to use ROS, as opposed to Gazebo alone:

• You have / will have a real robot which runs ROS and want to simulate it in Gazebo. Code which you write for Gazebo alone is not designed to be ported to a real robot.
• You want to have access to the whole ROS ecosystem while controlling your robots in Gazebo. ROS provides a lot of convenient tools for control, sensor data processing, navigation, etc, which may be beneficial to your project.

On the other hand, if you're planning on doing something simple and specific which won't be ported to a real robot, it might be quicker to write a pure Gazebo solution.