Here is it is
"Certain portions of testing such data validation etc can be efficiently verified by automation programs than humans in repeated way (humans make mistakes and often are terrible at repeated executions). By carefully identifying portions of application under test that could be "safely" checked (validated) by automation - you can speed up testing (you can run many test cases in parallel, in the night etc) through automation.
But beware - automation is a dumb and (humble?) servant - will do exactly what you ask it to do million times without cribbing - it does not have intelligence. A good tester can recognize something that is not in test script and looks like a problem. Automation cannot do this."