When a script performs the last step in your script, it will exit whether or not you include Exit Script. So you don't need it in the script you have posted here.
Exit Script has several key uses.
Sometimes you want a script to exit inside an If block in the middle of your script.
Sometimes your script is a subscript--it's called by a Perform Script step in another script, and you need to return a value to the other script.
Some Script Trigger performed scripts need Exit Script [False] in order to "cancel out" the event that triggered the script. (This only works with triggers that are tripped and perform a script before the triggering event is processed. See "Setting up Script Triggers" in FileMaker help to see a table of all triggers that shows which can use this script step to do that.)
Thanks Phil. So not needed where I put it but would be useful to get out of the script inside if block and in a few other cases.