When providing what you think is the correct answer, there is still some probability that you're mistaken. That probability could be 10^-10, but it can't be zero. And when answering "I don't know", you can still guess, and produce a probability that your guess was correct. Lumping all low probabilities of being correct into a single qualitative judgment of "I don't know" sometimes makes sense, but sometimes a concrete probability is useful so you should know how to generate one.