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.
This skill and Xing Zhen Hu + Judgment of Anubis is the ultimate lockdown strategy. Your opponent will not be able to use any spell or trap cards during the entire duel. Even if they play Twister from their hand to remove your trap you counter it with Magic Jammer / Judgement of Anubis!
This is why I love Tea <3 Or another excellent combo is the Unhappy Girl with Holly Guard which is a free ability. You run two of these traps to lockdown the trap zone then the Unhappy Girl and some defense position force change cards to also lockdown their monster zone. Totally evil!