Agile projects can do manual testing within an iteration, but the shorter your iterations, and the greater the number of iterations in your project, automated testing tools start to become more attractive. With lots of short iterations, you may not have time to do complete manual test coverage - especially if you are trying for full regression testing. In these cases, test automation is recommended and can be highly cost-effective. Plenty of low-cost or open source automated testing tools are available to help.