In games in general, any time there are more than two players, for any action, no matter how apparently self-defeating, it's usually possible to construct a scenario in which it is optimal. Your action might help another player, who then can block another player who is more of threat. Finding a reason for the move becomes even more of a possibility if you're engaging in collusion/trade with other players.
Even in a two-player game, there are edge cases where the scenarios you mention are optimal, especially your not buying a card case. If the reason you want the card is more to block the other player, rather than wanting the card for yourself, then reserving the card rather than buying it allows you to both block your opponent and save your gems for buying something that you actually do want.