The most obvious Benefits from automation are :
1. Speed and Accuracy – Automation testing is faster and more accurate than manual testing. It can be as much as 50 times faster depending upon the speed of the application to process information. Tools are much more accurate than manual test input. Research suggests that the average typist makes 3 mistakes for every 1000 keystrokes. Also, automation tool never tire, get bored, take shortcuts or make assumptions of what works.
2. Accessibility – Automation tools allow access to communication protocols, lower level objects of systems & operating system that manual testers can not access. This allows for a test suite with much greater depth & breadth.
3. Reusability – Once tests are developed, long term benefits are derived through reuse. Over the period of time, application changes and gains more complexity. The number of tests always keeps increasing as application matures. Testers have to constantly add new test scripts and need not write same scripts again or test the same area manually over and over again.
4. Manageability – You can group the test scripts according to functionality & prepare ready to use test suits. Depending on the change of code, you can pick up the test suite for testing. It is also possible to test different functionalities altogether or one by one using test suits.
5. Discovery of issues – Automated testing assists discovery of issues early in the development process, hence reducing the costs.
6. Repeatability – An automation suite provides repeatable process for verifying functionality of functional side and scalability on the performance side.
7. Availability – As they are automated, scripts can run any time ,any where that too unattended.
...Wow thats a real Ready reckoner...isn't it ?