If you prefer, think of the rational expectations framework as a benchmark -- much in the same way that perfect competition is a benchmark. It's not about whether the benchmark is wrong, so much as what the benchmark allows us to conclude when it is wrong.
The canonical example for rational expectations is the Barro-Gordon model of central banking (or more generally, the Kydland-Prescott model of policy-making), which essentially couches monetary policy in the framework of an infinitely repeated game.
The idea there was to consider the question of whether it is better to have a central bank that sets monetary policy "by discretion" or by adhering to a stated rule. In this case, the idealized assumption being made is that agents are rational and possess full information about the past behavior of the central bank (as well as the true value of pertinent measures like inflation & unemployment).
In the model, the bank's objective function implies a tradeoff between unemployment and inflation via the Phillips Curve. Both inflation and unemployment are costly, but setting both to the equilibrium value is not feasible.
In a one-shot game, the bank can improve the value of its objective function by "inflation surprises" -- basically, by injecting money into the economy without announcing it, so that in the short-run there is an increase to employment but no impact on inflation. But since realized inflation is a function of inflation expectations (because if people expect their money to be worth less tomorrow, they will spend more of it today, thus leading to increased consumption demand which drives up prices and makes the expectation self-fulfilling), in the iterated game setting rational economic agents will come to expect the central bank to generate "inflation surprises". What is interesting about the Barro-Gordon model is that this leads to an "inflation bias" -- realized inflation is always higher than the optimal value when the bank's actual policy targets don't match with the expectations they set out through statements, etc.
So rational expectations predicts that central banking done according to a rule is more efficient, because it keeps policy inflation in line with expectations and avoids this bias. In the literature one hears about "credible banks" and this is essentially what this is about.
Getting back to your question, is this used to "explain" forecast error? In a sense, yes: if the model predicts a certain value of inflation under rational expectations, and that value of inflation isn't realized, then one either questions the rational expectation model itself or (more likely) that one or more of the assumptions behind it is violated. Usually, this is the assumption of complete information, although one should also consider the possibility of additional shocks not accounted for explicitly in the model.