Automated testing may be executed continuously without the need for human intervention, they are easily repeatable, and often faster.
[5] Automated tests can, like any piece of software, be poorly written or simply break during playback.
A trained manual tester can notice that the system under test is misbehaving without being prompted or directed; automated tests can only examine what they have been programmed to examine.
When used in regression testing, manual testers can find new bugs while ensuring that old bugs do not reappear while an automated test can only ensure the latter.
One should consider the return on investment for automating any given test script, i.e. does the cost to build and maintain that script cost less than it would take to simply execute it manually.