When a system is developed, Systems Engineers test it to ensure that it accomplishes what it is supposed to. Often testing involves attempts to intentionally break the system to identify any problem areas. Existing systems also are tested regularly.
Once a system is launched, a Systems Engineer is responsible for monitoring its operation, troubleshooting problems that arise and keeping the system running smoothly. This includes creating reports and other documentation outlining your findings and solutions. As a Systems Engineer, you also provide support to users of a system.