Suppose that you go out on a date. The man/woman understands your personality immediately. They have a good grasp of your strengths and weaknesses, your wants and needs, and the way you think. I'll go a little further and say that this person is a decent human being and is interested in dating you. You are fairly certain they won't use this knowledge against you in an evil way, but of course they do know better how to ask you for things or to convince you. Assume slightly above-average looks, earnings, status, etc. for this date.
On the one hand, this person understands you so well already. You've just saved yourself months of awkward explanations, easily avoidable arguments, and embarrassing misunderstandings. They know what you like.
On the other hand, this means no mysteries and no illusions. The honeymoon period of the relationship won't be the same. They already recognized your jealous side, or your selfishness, or your tendency to be needy. They know what you are not.
Will you enter into a relationship with this person? You have to decide now, after this first date.