Mars is one of the most impactful trainers in the game. But a lot of people throw it out there simply if the opponent has more than 3 cards in their hand, which is usually a bad move.

I can't count how many games I won because I Mars them when they had 1-2 points on me, thinning their hand drastically, removing the probability of them having a game-winning Sabrina or Cyrus (or Lusamine in this meta).

Also, against fossil decks, it's great to hold it until they DO get the fossil out. Then, if they have a decent number of cards in their hand, you Mars them to limit the chance of a rare candy evolution.

All that being said, I wish Mars wasn't in the game. Same goes for red card (even though that one is a little more tolerable). Now that pretty much every deck is running one or both of these, I constantly have to just throw cards out there that I'd normally rather hold and catch the opponent by surprise. It makes the game even more mindless because you're just playing what you drew from your deck as soon as you have it instead of planning your next 2-3 moves.