This question is usually levelled against evolution or cosmology, by many famous Christian and Muslim apologists, and mindlessly repeated by amateur apologists.

In a general refutation of the scientific method (by strawmanning it), it is used as a means to discredit any scientific finding, by claiming the evidence is invalid because it was not directly observed (evolutionary processes, geologic processes, formation of stars and planets, &c). That is then used as "evidence” that atheism cannot be true.

Of course, the immediate counter is "were you there to observe the Immaculate Conception, life, and Crucifixion of Jesus” (for Christians) or "the life and times of Muhammed” (for Muslims).

The assumption is that Evolutionary Biology and cosmology are somehow equal to atheism. Evolution and cosmology are not atheism. Many Christians also accept the idea of evolution and the formation of the Universe; evangelicals not only attempt to discount atheism, but more liberal Christian denominations as well.

"Were you there” could also be used to argue against the American Revolution, the Protestant Reformation, the Roman Empire, or even the existence of Jesus.

Jimmy Carter said, "in many countries around the world—my wife and I have visited about 125 countries—you hear John Lennon's song 'Imagine' used almost equally with national anthems."