Here are some key reasons why we need to automate software testing:
Reusing the test scripts: When you want to execute the regression test scripts after every build it makes more sense to automate them. In case of testing web based application there is a more need to automate as the test suite has to be run on verious browsers like Internet Explorer, Firefox and other browsers.
It saves time: Running unattended automated test scripts saves human time as well as machine time than executing scripts manually
Better use of resource: While automated scripts are running unattended on machines, testers can do more useful tasks
Cost Saving: On test engagements requiring a lot of regression testing, usage of automated testing reduces the people count and time requirement to complete the engagement and helps reduce the costs