Hi Vaibhav,
It's similar than a normal dynpro. In a normal dynpro in SAP we have a PBO (Process Before Output) and PAI (Process After Input). So, GETTER method is like a PBO method, is executed before the ouput of screen, SETTER method is like a PAI method, is executed after an input in screen.
When we open a screen in SAP Web Client, GETTER methods are executed because the Web Client is showing the data.
When a user change a value and press enter or when a user press any button, SETTER methods are execueted because after input of user this kind of methods are executed.
But, as you ask, why GETTER methods are also executed after SETTER methods when a user press any button? This is because, after changing any data and press enter, the screen refresh the output and, before output, getter methods are executed.
Best regards.