Testing is necessary because software is likely to have faults in it and it is better (cheaper, quicker and more expedient) to find and remove these faults before it is put into live operation. Failures that occur during live operation are much more expensive to deal with than failures than occur during testing prior to the release of the software. Of course other consequences of a system failing during live operation include the possibility of the software supplier being sued by the customers.

Testing is also necessary so we can learn about the reliability of the software (that is, how likely it is to fail within a specified time under specified conditions).