If someone has proved to have an experience in X, and she has the same info/facts about the problem as you do (usually not), then she’s more likely to be right.
More likely, not always. This makes the whole difference.
1) Everyone is usually wrong, but it doesn’t matter. All that matters is who’s more likely to be right.
2) If she’s more likely to be right than you are, it doesn’t mean you have to follow the advice blindly!