The first use of that for me is for tabletop roleplaying games, in my case Dungeons & Dragons, when I am the Dungeon Master. A DM needs to make situational rulings, and he needs to present, or even create, the adventure. Those are all game design problems, so it helps to understand what the likely effect of something you say is on player motivation. If the halfling rogue in your party wants to jump on the chandelier, from there onto the ogre's back, and then cut the ogre's throat, both saying "you can't do that" and saying "okay, the ogre is dead" are probably bad solutions. The good solution is to require one or more checks, and on success allow some sort of bonus, like an automatic critical hit. You need to understand the game design of combat to be able to balance the difficulty of the checks with the benefit of the bonus that could be gained. You would want to reward the player from doing something beyond "I attack", but don't want to create a precedent for an easy and repeatable overpowered action.

But I also use my knowledge of game design to modify games, some people might call it cheating. Cheating is a tricky business: Some video games have built-in cheat consoles, that for example let you activate a "god mode", in which you are invincible; not a good idea in most cases, as it could easily kill all the fun of a game. But when you understand the design of a game, you also understand that it is possible that some things are in the game purely to make the game longer. If a game element is clearly a grind, I don't feel bad about modifying it. I recently saw a YouTube video where somebody said that he didn't like the Tainted Grail board game, because the grind necessary to re-light the menhirs took the fun out of the game for him. I haven't started playing Tainted Grail yet, it is on my board game shelf of shame, but I already know how it plays. The menhirs are important enough for the game that I wouldn't remove them, but I wouldn't feel bad to change the rules to make them stay alight longer, giving the players more time to explore. Tainted Grail is a €100+ (and even more expensive in the US) board game; if there is a rule in it that makes the game not fun for you, would you rather change the rule, or give up on playing the game?

I feel the same about some video games, for example I remember in Assassin's Creed Odyssey hating the collection of wood, because it was a grind that for me wasn't the part of the game that I was playing it for. But there is software like Cheat Engine or WeMod that allow me to cheat and modify such game elements. The trick is to use your knowledge of game design to understand what the actual game and challenge is, so as to not destroy that, and what the grindy content is where there is no harm in skipping it.

Even more tricky than eliminating grind elements is the modification of difficulty. Unless a game is PvP (and I rarely play PvP games), the challenges in both board and video games can be somewhat arbitrary. You never want to modify those to be completely trivial, but different people have fun at different difficulty levels. Video games have made great advances in game design over the last decade, so for example games that have a lot of story and exploration these days often offer a "story mode", in which the challenge is minimized. For the people who play the game for the story and exploration that might be the perfect solution, while others play for the challenge and can instead choose one of the harder modes. With board games, variable difficulty level is not as common; but it is far easier to modify the rules of a board game than to hack a video game. You just need to understand enough of the game design to know whether changing for example the healing potion to give you back one more health is something that would make the game slightly easier or whether that would make the game trivial.

The purpose of a game is to have fun. It is possible that a game isn't fun, either because of a flaw in game design, or because of a game design element that might be fun for other people, but isn't for you. We live in a world with such a huge supply of games that there is a temptation to just move on to the next game if something isn't fun to you. But in some cases I find it worth while to modify a game to make it fun, rather than giving up on it.