People say that the truth comes out some how whether its soon or years, months even days later. If you keep a secret then the it can come out in the future just by holding it inside for a long time. The truth eats you up and in the future it can eat you up because it could become a dark secret.
As for the present lets just say you can't keep the secret inside its like you don't want to hide anything, you want to remain an open book, remain trustworthy, and not a liar. The truth has to come out as sooner than later. Additionally, you don't want the ones that involve the secret to get hurt all because of you not speaking soon enough.
The past... Well you make mistakes in the past and you don't necessarily want to make them again so want to be able to be forgiven, you want to erase those bad sins away; wash away the past. The main way to regain trust is to start by telling the truth so that others know that you are loyal again. Beginning to tell the truth after one has messed up is almost like getting a fresh start in life.