So the firm has to forecast the prices it's going to sell at in the future. It also has to forecast what competitive products are going to be around. It has to forecast the prices of the inputs it uses that are complementary with their capital, and so on. Thus, one of the big jobs of business is to forecast the future. Economists want to study what business does and to have some principle for understanding how business or consumers or any economic agent forecasts.
Rational expectations assumes that people use all the information they have as well as they can.
This is just an application of the economist's general perspective in analyzing people's choices. We assume that people do the best that they can, that they behave in their own interests. In the decade since the beginnings of these ideas, I'm sure there's been an evolution. In the '70s when we were doing work here at the bank, it was very controversial. At that time there were still plenty of defenders of the old Keynesian models that rational expectations really threatened in a very profound way.