It is an aspect of being one with your true Buddha nature because only when you are free of the control of your mind can you know the truth, because only then can you observe yourself and the world around you with dispassion, free of labels, free of emotion, free of the intervention of your mind. You see reality.
When you are not free of the control of your mind, you see everything through the lens of your mind; what you observe with your senses - sight, sound, smell - which are themselves neutral, is interpreted and refracted by your mind's life experiences, emotions, and perceptions into your mind's biased version of reality.
There is an old saying that the truth will set you free. It's not always what you learn about the truth – although often that will in fact be the case, especially about the past – but that since you can only see the truth when you are free of your mind's emotions and perceptions, that is what sets you free.
Seeing yourself and the people and world around you as things truly are is critically important for making sound judgments, decisions, as you go through life. Depending on how far you are along on the path, to what extent you have renounced your mind and are at one with your true Buddha nature, your mind may still be strong enough to counter the truth you see and make you adhere to its view of things. Certainly one must be aware that the mind is always out there, waiting for an opportunity to assert itself, no matter how far along you are on the path.
As for speaking the truth to others, only when you are one with your true Buddha nature will you have the courage to do that. Why does that take courage?
Because most people do not want to hear the truth – about anything. They see the world and themselves through the lens of their own mind, they have their own personal version of reality, and they do not want to hear the truth because the truth is threatening in many ways. The truth is challenging. It rocks the boat.
Having the courage to speak the truth does not mean that you speak the truth to everyone in every situation like you're on a mission. One has to judge when the truth is needed and when the truth can be spoken without inflicting harm (although there are situations where speaking the truth is so critically important that even if it harms someone, it must be done). In one sense, one can say that the truth is always needed, but without question in some situations the need is more urgent than others.
You must be aware, however, that by speaking the truth, you may well be shunned by the very people or their friends that you a trying to help. Again, people just don't want to hear the truth because typically they will hear something they don't want to acknowledge.
So speaking the truth is often an isolating experience. That's why it takes courage.