Basically the Post Value action will be used to post your field value into the clipboard memory on the specified event trigger. So if you want to post the current value of the field into clipboard memory to use it for further processing you can use this Post Value action on the controls.
Additionally the Pega help says the below information,
Posts the property value to the clipboard and refreshes the cell's contents.
Post Value is triggered by Change or OnBlur of the field and prior to any other event (such as a click action) initiated from a control, or from the calculation of expressions.
For example: a pxButton control refreshes another section using an activity that requires three updated properties before the control can update the target section. When the user clicks the button, the properties are posted to the clipboard and are synchronized with the activity. The properties are already on the clipboard, in most cases, when the button is clicked.
Hope this information might give you more knowledge about its usage.