Truth is important.
It may sound obvious. That's what we've been taught since childhood. Lying is bad; honesty is good. But our parents and teachers are not infallible. They could have been wrong. Lying and bullshitting certainly seems quite prevalent in the behavior of those at the top.
Also, a major component of philosophy is to check your assumptions. Wrong assumptions lead to wrong conclusions, which is bad philosophy. So, are we sure that the truth is a good thing?
My answer is yes. First for practicality. If you're pursuing another goal, the truth will only help you get there. If you're trying to make people happy, you need to know what will actually make them happy. If you do something that you believe will make people happy, but are mistaken, you will achieve the exact opposite of your goal. And this is true of any goal. Even if your goal is to dissemble and mislead, you'll be able to do it better if you know the truth.
But beyond that, I feel that truth is good in and of itself. I can't really articulate why. It's a non-rational preference, the same way preferring pleasure to pain or happiness to unhappiness is non-rational. And I feel it's a very important preference, of the magnitude of pleasure or happiness.