Are we really down to “but the book says…”
Okay.
The book says, “You might choose to make a roll for a player because you don't want the player to know how good the check total is. For example, if a player suspects a baroness might be charmed and wants to make a Wisdom (Insight) check, you could make the roll in secret for the player. If the player rolled and got a high number but didn't sense anything amiss, the player would be confident that the baroness wasn't charmed. With a low roll, a negative answer wouldn't mean much. A hidden roll allows uncertainty.” (DMG, p235).