According to the ISTQB Glossary, regression testing is required for what purpose?

Regression testing is conducted to ensure that modifications made to a software application, such as enhancements or bug fixes, have not introduced new defects or adversely affected existing functionalities. The primary objective is to confirm that the software behaves as expected after any changes. This is essential for maintaining the software's integrity and reliability, especially as systems grow more complex over time.

When modifications are implemented, there is a risk that new code might inadvertently disrupt other parts of the application that were previously functioning correctly. Therefore, regression testing focuses specifically on re-executing previously completed tests to check if the older functionality still operates as intended after the recent changes have been applied. This process ultimately aids in identifying any new issues that may arise due to the modifications, assuring quality and promoting confidence in the stability of the software post-changes.