Giving your opponent a spell or trap card will make their field smaller for other cards. If you give them a card with a surprising negative effect like Card of Sanctity, they might lose the duel just by activating that.
|Select 1 card Set in your Spell & Trap Zone, and force it on your opponent. Only the player that activates this skill can view this card. This skill can only be used once per Duel.|
If you don’t force your opponent to use that card, they can simply choose not to activate it. It is not a competitive skill because it is limited to only one activation per duel, so even if you build a deck around it, it is not guaranteed that you will manage to take good advantage out of it.