Estimated execution plan is the result of query optimizer, it is generated even before the query execution. Hence, it might have some or more changes when compared with the actual execution plan, but in most cases the actual and estimated execution plan remain almost the same.
As a DBA, many times you might come across a situation where you come to the office in the morning, somebody comes to you complaining that one of the page is taking a long time to display data which was working just fine and fast some days back. What would you do in the first step? How do you determine where the problem is?
This is the time where the estimated execution plan comes into picture. You take the query or stored procedure from the page which is showing the data very slowly (though there could be other reasons too, but let us assume that it is because of SQL Server only, as of now.) and check the execution plan of the query or stored procedure in order to...