Didn't need to do something VS. Needn't have done something

Do you know what the difference between these two expressions is?
Can you explain it? You can use Russian here.

If you aren't sure, read these two small dialogues which I made to illustrate the point:

Dialogue 1:

Brian: Let's have dinner tonight. I know a great French restaurant.
Justin: But it's Friday night! All tables will be reserved! We need to book a table.
Brian: No, there aren't usually so many customers there. I'm sure we don't need to book a table.
Justin: No, I want to be sure. Let's book it anyway.
Brian: OK. I'll do it.
------ some time later they come to the restaurant -----
Brian: Look! There are so many free tables! We needn't have booked one/ We didn't need to book one. - both expressions are possible

Dialogue 2.

Brian: Let's have dinner tonight. I know a great French restaurant.
Justin: But it's Friday night! All tables will be reserved! We need to book a table.
Brian: No, there aren't usually so many customers there. I'm sure we don't need to book a table.
Justin: OK. Let's just go there.
------ some time later they come to the restaurant -----
Brian: Look! There are so many free tables! I told you we didn't need to book one. - only this expression is possible
Justin: Yes, you were right. We didn't book it anyway.

Ok. So do you get the point now?