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.
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.