A CircleCI developer is primarily responsible for integrating and automating the software development process. They write code for CircleCI, a continuous integration and delivery platform, to automate building, testing, and deploying applications. A CircleCI developer streamlines the workflow by automating tasks like code integration, testing, and bug detection, making it easier to catch and correct issues early. They also ensure that the codebase is consistently updated and in sync across different environments. Additionally, they monitor and analyze the performance of the CI/CD process, troubleshoot issues, and work on improvements to optimize the development pipeline.