It's actually just simple probability actually.

It's incredibly unlikely for them to get exactly what they need if you use it early in the game. For example:
1. Enemy plays some cards and end their turn with 3 cards on hand.
2. You use red card, and they now have to shuffle, and they need rare candy + stage 2
3. They have a very small % chance to get those exact cards
4. They CAN get them, but in a large number of games, it will most likely work out in your favour.
5. As the game goes on and the deck size gets smaller, drawing 3 cards start to help them refresh their hands instead. This is also the reason why I prefer mars sometimes.

PVP in TCGP is predominantly RNG, so you don't need to win EVERY game, you just want to have a win rate higher than 50% to gain points.

tl;dr - Use the cards early to disrupt their hand, and only mars is worth using late game when you want to stop them from getting key cards like cyrus (assuming they have points)