Once the system has been developed it must be tested to ensure that it is working as expected and that there are no obvious faults.
Faults are otherwise known as software bugs. These are basically mistakes in the software code. No matter how careful coders are or how much time they take, they are only human and mistakes will inevitably occur at some point.
Testing aims to pick up these software bugs before the system is released. It will also ensure that the system is reliable and robust enough to be used for its intended purpose.