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Answer:
Some atheists say we shouldn’t be surprised the physical laws of the universe (such as the strength of gravity) are fine-tuned for life, because if they weren’t, we wouldn’t be here to appreciate that fact. These critics say a finely tuned universe is “necessary” because of an anthropic coincidence: we shouldn’t be surprised that we observe a life-permitting universe because those are the only kind of universes we would observe.
The anthropic principle does explain why we don’t observe life-prohibiting universes (because we’d be nonexistent). However, it doesn’t explain why we observe such an incredibly unlikely life-permitting universe. To borrow an example from John Leslie, that would be like saying we shouldn’t be surprised fifty trained marksmen shot me and all of them missed, because if even one of them had killed me, I wouldn’t be here to appreciate that fact.
Just as there must be a reason the entire firing squad missed, there must be a reason that the universe is so improbably fine-tuned for intelligent life.