Richard Dawkins explained it best here:
The standard methodology of creationists is to find some phenomenon in nature which Darwinism cannot readily explain. Darwin said: “If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.” Creationists mine ignorance and uncertainty in order to abuse his challenge. “Bet you can’t tell me how the elbow joint of the lesser spotted weasel frog evolved by slow gradual degrees?” If the scientist fails to give an immediate and comprehensive answer, a default conclusion is drawn: “Right, then, the alternative theory; ‘intelligent design’ wins by default.”
Notice the biased logic: if theory A fails in some particular, theory B must be right! Notice, too, how the creationist ploy undermines the scientist’s rejoicing in uncertainty. Today’s scientist in America dare not say: “Hm, interesting point. I wonder how the weasel frog’s ancestors did evolve their elbow joint. I’ll have to go to the university library and take a look.” No, the moment a scientist said something like that the default conclusion would become a headline in a creationist pamphlet: “Weasel frog could only have been designed by God.”
I once introduced a chapter on the so-called Cambrian Explosion with the words: “It is as though the fossils were planted there without any evolutionary history.” Again, this was a rhetorical overture, intended to whet the reader’s appetite for the explanation. Inevitably, my remark was gleefully quoted out of context. Creationists adore “gaps” in the fossil record.
Many evolutionary transitions are elegantly documented by more or less continuous series of changing intermediate fossils. Some are not, and these are the famous “gaps”. Michael Shermer has wittily pointed out that if a new fossil discovery neatly bisects a “gap”, the creationist will declare that there are now two gaps! Note yet again the use of a default. If there are no fossils to document a postulated evolutionary transition, the assumption is that there was no evolutionary transition: God must have intervened.
The creationists’ fondness for “gaps” in the fossil record is a metaphor for their love of gaps in knowledge generally. Gaps, by default, are filled by God. You don’t know how the nerve impulse works? Good! You don’t understand how memories are laid down in the brain? Excellent! Is photosynthesis a bafflingly complex process? Wonderful! Please don’t go to work on the problem, just give up, and appeal to God. Dear scientist, don’t work on your mysteries. Bring us your mysteries for we can use them. Don’t squander precious ignorance by researching it away. Ignorance is God’s gift to Kansas.