MSSQL Execution Plan

I just found out that there two (2) types ways to calculate the execution plan: 1.) Estimated Execution Plan and 2.) Actual Execution Plan.  The screenshot below shows where to toggle it -- MS SQL Server 2005 Management Studio

So what is the importance?  If you happen to have a query that doesn't output a result, for some unknown reason, then the Estimated Execution Plan might help in pinpointing the problem.  This gives you an idea where to look at first instead of waiting for the actual result to come out which may not.