Action is like a function. as you use function whenever is required this is like the same.
Other use is you can also devide your application in logical sections lets say for ex. you are testing gmail then you can devide your test script in login, checkmail, sendmail, logout actions.
Other use is you can also devide your application in logical sections lets say for ex. you are testing gmail then you can devide your test script in login, checkmail, sendmail, logout actions.