Are “To be honest with you” or “to tell you the truth” mostly used to share honest answers or to cover up a lie?

It is commonly used as a way to prepare someone for an honest and potentially harsh truth. For example, imagine you know someone who is far from being a decent person. They lie constantly, abuse their partner, and even steal from innocent children. Now, you pretend to be their friend out of pity, but deep down, they disgust you. One day, they express surprise that someone doesn't like them and ask why. At that moment, you can't hold back anymore and decide to tell them exactly what you think. You might start by saying, "To be completely honest, you're...," or "Honestly, you're a horrible person." This kind of situation calls for the use of these words to allow the person to prepare themselves emotionally.

However, some more timid individuals also use these words to soften their evaluation of someone's character. For instance, let's say there's this guy named Tiffany (don't ask, just go with it). Tiffany is constantly boasting and is incredibly full of himself. Nobody wants to be around him because of his inflated ego. He also enjoys murdering people and eating them, although that seems rather minor when compared to his ego. One day, you come across Tiffany crying in the mall's food court. Curious about his problem, you ask him what's wrong. Through tears, he confesses that nobody wants to be his friend and asks what he's doing wrong. In response, you say, "To be fair/to be completely honest with you, sometimes you can come off as a little too much. Some people might think you have an overly inflated ego... sometimes. Also, the whole murdering and eating people thing might make some people uncomfortable too. I'm just throwing some suggestions out there. Honestly, I have no idea why people don't want to hang out with you. You're amazing."

So, the phrase is quite versatile. It serves as a warning for forthcoming honesty or as a way to sugarcoat the truth. Its usage depends on your personality and the personality of the person you are addressing. If the person lacks understanding, then be honest. If the person has a tendency to react violently to criticism, use it to soften the blow. And if you're using it on your wife or girlfriend, make sure to add, "To be completely honest, dear, no, that dress looks absolutely stunning on you," or "Honey, you didn't burn the meatloaf that bad, in fact, to be brutally honest, I've never had crunchy meatloaf before and it was surprisingly good! It added a fun element to the dish. It was an incredible experience." However, if you answer any differently, she will make Tiffany look like Mother Teresa.