Some argue like this:
Surely we can know that something exists, yet also know that its existence is highly improbable, improbable enough to demand some sort of explanation?
Isn't precisely this true of the existence of the universe?
The playing cards
Here's a Swinburne-type illustration of the general point. Suppose I am asked to guess each one of 52 cards, one by one. If I ever get one wrong, my brains will be blown out.
I start guessing, and amazingly, I get all 52 cards correct. Now you may say, "What's so improbably about that? After all, the probability of you getting them all right is 1, as you wouldn't be here otherwise would you?"
But of course, there's a sense in which something deeply improbable has happened. So improbable, in fact, that it would be reasonable for me to suspect this result wasn't just a matter of chance.
Some of those who favour fine-tuning arguments for the existence of God argue that, similarly, the fact the fact the we do exist does not show that there isn't something extraordinarily improbable about the universe, by chance, being just right for life. So improbable, in fact, that we can reasonably suppose that its "fine-tuned" character is not an accident, but the result of deliberate design.
The firing squad
Here's another classic example of the general point. As a condemned spy, you are put before a firing squad of twenty expert marksmen, who load aim, and fire at your heart from close range.
Amazingly, they all miss. You feign death, and survive.
Pure luck that they all missed? Possibly. But highly unlikely.
Far more likely that the miss was deliberately arranged. It won't do to now say, "But their all missing is not amazing at all. It's wholly unremarkable. After all, had they not all missed, I would not be here to ponder my luck!"
In the same way, its argued, we can ponder the improbability of the universe (of its being "fine-tuned" for life, etc.), despite its epistemic probability now being 1. We can't dismiss this alleged improbability by saying "But it's not improbable at all - after all, if the universe had not been just right for life, we would not be here!"
One of the commentators on Why There is Something Rather Than Nothing below, does make this move. It's not uncommon. But I don't buy it.