When attempting to produce a correct program, what is the point of developing a test suite?

When attempting to produce a correct program, what is the point of developing a test suite?

To provide a small set of inputs as a test to verify that a program will be correct for all inputs.

To test the outputs of a program against known values to verify that the math coprocessor is performing properly.

To test the code syntax for possible errors such as undefined variables.

To modify the running program's variables and verify that the program will still function, even if the code changes.