Code reviews can be used both with CI systems and without them. Their main purpose is to double-check each change introduced to the code to make sure that it is correct, that it fits the application's architecture, and that it follows the project's guidelines and best practices.
When used without CI systems, it is often the reviewer's task to test the change manually and verify it is working as expected. CI reduces this burden, letting software developers focus on the logical structure of the code.
