Saving search criteria as Email Alert serves 2 purpose:
1)	Your search criteria is saved and you can simply click on search button on Email Alerts screen (require's Login) that will use same search criteria to find properties. This way you dont have to enter search values again & again to search advertisements.
2)	And the main feature is that an Email will be sent to you automatically whenever a new property is posted or existing property is modified that meets your criteria. One Email per alert per day will be sent, only if at least one property meets that Alert's criteria. If you don’t want Email to be sent, simply disable the Alert or you can also delete them at Alerts screen.