A common interview question at Microsoft for testing positions is, "describe the advantages of test automation compared to manual testing." I advise canddates to answer with "SAPES": test automation has better Speed, Accuracy, Precision, Efficiency, and Skill-Building. The Speed part is pretty obvious — you can run thousands of tests very quickly but with manual testing you’re lucky to do 200 a day. The Accuracy part means that automation is not subject to as many errors as manual testing, such as incorrectly recording a test case result. The Precision factor means that automated tests run the same way every time but that manual tests often will be slightly different when different people perform the tests. The Efficiency part means that automated tests can run overnight, or if run during the day it frees you for more interesting testing that must be done manually. Finally, the Skill-Building factor means that when you write test automation you are simultaneously enchancing your technical skills, whereas manual testing is often mind-numbingly boring.
Test automation does not replace manual testing, it just handles the boring, repetitive testing. Test automation has a significant price in terms of both time and money and not all test tasks should be automated. We usually say that test automation allows you to perform more thorough testing rather than say that automation saves time and money (because it often doesn’t).