You can never be sure about the quality of your in-development software unless you test it first. Conducting a thorough analysis of whether your software meets all the requirements, whether it is secure, responsive, complete, and easy to use is crucial to avoiding technical debt and making sure your software is well received after the release. And this is exactly what testers do for you.
Software testing is a process of investigation. A tester pushes your software through a great variety of tests to catch any hidden errors, unpredictable behavior, or functional inconsistencies. After every test, a tester files a detailed report that helps developers fix all the exposed issues, maintain the software error-free, and make sure it runs as intended.