When calculating the value of perfect information, why do we need to use the probability if we are assuming we are obtaining 100% accurate information?
Because when we pay for the information (market research) we do not know which of the various possibilities they are going to tell us will happen!
If there is (for example) a 20% probability of one thing happening, then there is a 20% probability that the market research will tell us that it is going to happen.
I do suggest that you watch my free lectures on this!