The future perfect is used to describe an action that is not complete now, but will be complete at some definite point in the future.
First consider the present perfect. Take a sentence like "I have written a letter." What this is means is this: there now exists a letter written by me. The focus of the present perfect is on the result of the action, and the result is in the present.
The future perfect is the same, except that the (expected) result is in the future. "At 10 o'clock I will have written a letter" means that at 10 o'clock there will exist a letter written by me.
That's different from other future forms:
"I will write a letter" = I have just reached a decision to write a letter.
"I am going to write a letter" = I have planned to write a letter.
"I am going out with Jane this evening" = I have made all necessary preparations to go out with Jane this evening
None of these things says anything about when the action is to be complete, only that the action is to take place. The future perfect focuses on the result of the action. The action could take place at any time, but the result will be evident at the stated time in the future:
"By the year 2106, George Bush will have died." = although we don't know when Bush will die, we do know that in 2106 there will be a grave or an urn somewhere with him in it.
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