As an example, they may forecast that 30% of the areas your city will report rain tomorrow. They may have an high level of confidence in this prediction (and verify it to be correct after the fact). It may still not be very useful to you, since the one outcome is so different to the other.

That said, I agree with your that the predictions should be presented in a more scientific manner. In fact, one irritating pattern is that rain probability and the predicted amount of rain sometimes get combined into a single percentage, which no longer represents neither the probability, nor the amount. It's some kind of weird combination of the two.