Perhaps a very keen and alert QA/developer will catch the bugs, but even the best of us sometimes overlook or miss things.
Automating tests can help you find some additional bugs that you can overlook during manual and exploratory testing, particularly in the field of web applications, for browser differences:
- unexpected behavior of some browsers
- different formatting in some browsers
Those two cases have happened to me. Would have missed them if not for automation, due to oversight in visual inspection and not regression testing everything across all browsers. It was actually easier to spot by automating to run for all browsers and catch it failing when running against the problem browser(s).
Though I assume some of you are already aware of this benefit…