The purpose of this idiomatic logic bomb is to establish a fervent belief in the truth of the statement at hand.
~~~The unreality of the falsity emphasizes and gives validity to the subjective "truth" that you are trying to put across in your statement.~~~
For example, if we went fishing and I stated, "Hey, Nice catch! Looks like it is at least three pounds."
You might respond, "Three pounds? Look at this big fellow; he is four pounds if he is an ounce."
You made the statement to emphasize the surety of your estimate.
This "comparison" between a false thing and a true thing uses the falsity to emphasize the truth.
"That man is sixty years old if he's a day."
Similar uses can also be done to emphasize a falsity:
"If she's a virgin, then I am a faery princess."
Hence, the unreality of the falsity emphasizes and gives validity to the subjective "truth" that you are trying to put across in your statement.