People tend to believe in others being honest, and it can be a boon for a person trying to hide an awful past. They could be single and seeking a partner in life, so telling the person what they want them to know is a way of hiding behind falsehoods. They can say whatever they want, but it will come back to haunt them if a relationship develops. Discovering the truth can be devastating when a person is emotionally involved with someone who told them a pack of lies when they first began dating.
There are many things people do in life that they want to forget or dismiss, and there is not necessarily anything wrong with that. They could leave out the fact they failed a driving test eight times before passing, but the fact they are now licensed to drive lessens the value of what they left out. For those with more serious items to hide, telling everything on the first date is not necessary. They will have to own up to it before the relationship becomes serious.
Involvement with another person on an emotional level gives them the right to know who they are with, and finding out from other sources is where the heartbreak begins. A person who spent time in prison might want to keep it a secret, but the one they are planning to build a future with deserves to know. Finding out when a stranger comes to town could turn the relationship sour in the time it takes to confirm the fact.
There is now a galaxy of information available online, but people still need a reason to go through it. Some might have a feeling their partner is not telling the truth, but others could find their friends or family have their own suspicions. Failure to be honest could eventually split up two people who might have had a great future together.