Regression testing has the task of verifying that a modification performed on a part of the product does not create problems to other parts not changed.
The use of automated test procedures allows you to perform more accurate checks as well as reducing the level of human resources in repetitive tasks.
“Also as a consequence of the introduction of new bugs, program maintenance requires far more system testing per statement written than any other programming. Theoretically, after each fix one must run the entire batch of test cases previously run against the system, to ensure that it has not been damaged in an obscure way. In practice, such regression testing must indeed approximate this theoretical idea, and it is very costly.” — Fred Brooks, The Mythical Man Month.