This is a fun game! I'm not going to bother spoilering these since there are so many already.
You're on the Gardens plan and your opponent is playing Possessions. Silver is probably good enough for you, and Gold might help them too much. (Explorer is actually good on the gardens plan, and gardens is probably good against Possession, so this isn't even that unreasonable!)
Your deck is made out of Silvers and Wishing Wells, and you want to keep wishing for Silvers and being right. (This is a stretch, because I think that's a pretty terrible deck, but once you're there it could be the right play)
In a similar vein, you need exactly 2 money and aren't going to reshuffle before the end game, and you know they have Smugglers and are going to reshuffle (or have Watchtower)
You're playing the Explorer solely so that you can have another card you don't care about in your hand so that you can play your Steward to trash your Curse, and you'd prefer not to let your opponent know that your Province is in hand (because of your Contraband, or because you don't want them to know you missed on Tournament, or because you want them to be scared that you'll Salvage it later...) Or, on isotropic where they'll know about your province anyway, because there's only 1 Gold left and you want to buy it.
My favorite:
You have enough money already, you are planning to buy exactly 5 cards later this turn under Watchtower or Royal Seal, Adventurer is in the spread and you really don't want it, only good threes are in the spread, and you know that Swindler is in their hand (due to Cutpurse/Bureaucrat/paying attention and it's their last hand)