In Magic: the Gathering and other popular card games, advanced players have developed the notion of a "win-more" card. A "win-more" card is one that works very well, but only if you're already winning. In other words, it never helps turn a loss into a win, but it is very good at turning a win into a blowout. This type of card seems strong at first, but since these games usually do not use margin of victory scoring in tournaments, they end up being a trap-- instead of using cards that convert wins into blowouts, you want to use cards that convert losses into wins.
This concept is useful and important and you should never tell a new player about it, because it tends to make them worse at the game. Without a more experienced player's understanding of core concepts, it's easy to make mistakes and label cards that are actually good as being win-more.
This is an especially dangerous mistake to make because it's relatively uncommon for an outright bad card to seem like a win-more card; win-more cards are almost always cards that look really good at first. That means that if you end up being too wary of win-more cards, you're going to end up misclassifying good cards as bad, and that's an extremely dangerous mistake to make. Misclassifying bad cards as good is relatively easy to deal with, because you'll use them and see that they aren't good; misclassifying good cards as bad is much more dangerous, because you won't play them and therefore won't get the evidence you need to update your position.
I call this the "win-more problem." Concepts that suffer from the win-more problem are those that-- while certainly useful to an advanced user-- are misleading or net harmful to a less skillful person. Further, they are wrong or harmful in ways that are difficult to detect, because they screen off feedback loops that would otherwise allow someone to realize the mistake.
The real danger of the "win-more" concept is that it's only barely different than making choices that turn an advantage into a win. You're often put in a place where you're somehow ahead, but your opponent has ways to get back in the game. They don't have them yet - you wouldn't be winning if they did - but the longer you give them the more time they have.
For a personal example from a couple years ago, playing Magic in the Legacy format, I once went up against a Reanimator deck with my mono-blue control deck. The start was fairly typical - Reanimator trying to resolve a gigantic threat to win, while I played many counterspells and hit him with some Vendilion Clique beats. My opponent ended up getting an Iona out (naming blue, obviously), but went down to exactly one life to do so. This was very, very awkward for him, since he couldn't attack with the Iona, activate fetchlands, or use the alternate cost of Force of Will. But, I had outs - Powder Keg (7 copies of keg/ratchet bomb) and waiting 9 turns, or Vedalken Shackles (3 copies). So I stayed in, and got as many draw phases as I could, and lucked out with a Shackles topdeck, followed by being able to play blue spells and winning the game.
Anyhow, my point is that cards that help you only when you're winning can turn wins into losses. Your opponents can have outs, and it's a good idea to take those outs away. If you don't, then sometimes your opponent will pull exactly what they need to do something ridiculous - say, dealing with a card that keeps them from playing 28 of their 38 spells, and seven of the ten spells they can play take 9 turns to do anything about it.
"Win-more" is definitely the wrong word to describe this concept. I think a better choice is calling it a "close-out" or "finishing" card. The point of these is to make sure that you win when you have an advantage. It also tells you that you don't want too many of these - many decks run just one or two copies. Dredge, for instance, runs a single Flayer to turn having their deck in their graveyard into a win. My mono-blue control deck ran two Sphinx of Jwar Isle (there were essentially zero answers for him in the meta, and I've stolen games with him. That said, one copy would be an Aetherling if it was printed at the time).
Replacing a card with a finisher means that you'll take fewer leads, but win more games while ahead. Sometimes the right number of finishers is one - when Dredge has a lead, it's got access to all or most of the cards in their deck. Sometimes it's more - my mono-blue deck would run between 2 and 6, depending on how I felt about Jace at the time. Often it's zero, and your game plan is to win with the cards that got you ahead in the first place.
I think it's important to make the difference between finishers (such as Sphinx of Jwar Isle) and win-more cards (such as Nomad's Assembly).
A finisher is something you use to end a game quickly. A win-more card is a card that only helps you if you are already ahead.
The Sphinx ends the game in 4 turns no matter what. It's a good win-condition. Nomad's Assembly is something that only helps you win if you already have a lot of creatures.