The main role of a quality assurance tester in software development is to ensure that software products meet specified requirements and are free from defects. They conduct various types of testing, including functional, performance, and usability testing. Quality assurance testers design test cases, execute them, report bugs, and work closely with development teams to address issues before release. Their goal is to enhance the quality of the software, ensure a seamless user experience, and prevent costly errors or failures in production environments, thereby contributing to overall product reliability.