Checks if bug has been addressed: The first objective in bug-fix testing is to check whether the bug fixing has worked or not. Therefore, you run exactly the same test that was executed when the problem was first found. If the program fails on this testing, it means the bug has not been fixed correctly and there is no need to do any regression testing further.

Finds other related bugs: It may be possible that the developer has fixed only the symptoms of the reported bugs without fixing the underlying bug. Moreover, there may be various ways to produce that bug. Therefore, regression tests are necessary to validate that the system does not have any related bugs.

Check effect on other parts of program: It may be possible that bug-fixing has unwanted consequences on other parts of a program. Therefore, regression tests are necessary to check the influence of change in one part on other parts of the program.