It depends on the game.

But in general, you play the Trump card to bring home the winnings.

Moving from the idea multiple meanings exist, I see the following:

1. WE live in the era of Trump. Meaning, Trump is a set of cards being played, where Donald Trump embodies the "Trump Card" dominating the game.
2. In individual cases, a Trump card is played at the point to bring home the winnings of a round. And this is more related to timing than just the final round in triumph showing up with a Jack of Trump.

I liken it to: Whist, Boer'n Bridge or Klaverjas.

There are a couple of reasons to play A Trump Card:

1. The other party shows a color that you cannot meet, and so you have to play a Trump Card
2. The other party plays a Trump Card but you must decide to either take this round by showing a higher Trump Card, or if you cannot, stuff that round with a BS card, and wait for a better opportunity to play that card.
3. You open, based on the hand you were dealt, to draw out the best Trump cards of the other party, hoping to end up with the best Trump cards in the round, meaning you can at least bring home 3 rounds, especially the last one.
4. A variation on 3 is to draw out the Trump cards so you other cards remain untouchable. The best round win, is one where you finish it on a normal card, preferably with a suit: like 4 times 7, 4 times 8, etc.
5. Best play in a round is either a march (meaning you win all 8 rounds in a set) or ... playing in such a way, that the playing opposing party is incapable to meet the threshold of 82 point and goes utterly and absolutely fucking wet!