Consider a card like Sangan, which involves searching for a monster of a specific criteria and putting it in your hand (emphasis mine).
If this card is sent from the field to the GY: Add 1 monster with 1500 or less ATK from your Deck to your hand, but you cannot activate cards, or the effects of cards, with that name for the rest of this turn. You can only use this effect of "Sangan" once per turn.
Similarly The Shallow Grave involves searching for a monster and placing it face down.
Each player selects 1 monster in their Graveyard and Special Summons it in face-down Defense Position.
In both cases, the card I chose is now in a location where my opponent cannot verify that I picked a valid card (in Sangan's case, they can't look at my hand to confirm that I have chosen a Monster card with 1500 or less ATK). This makes me wonder: do I have to reveal the card I have searched for? Or are my opponents just supposed to trust that I picked a valid card?