A new turn begins: you draw up to your hand limit and survey your new cards. A three-ops card, yes, this is just what I needed. A card that might diminish your losses in battle, okay, I can work with that. That powerful special event you were hoping to see on the previous turn, well, better late than never. And then, there's that Mandatory Event card you absolutely did not want to see on this turn, or on any turn. It's the card that will undo your modest, hard-fought gains, the card that will destroy you, that will cheerfully assist your opponent in looting your corpse.
Several years back, there was a CDG that came out that was, to put it charitably, flawed. I'm not going to name-and-shame, but its victory conditions were pretty wonky and the flavor text trafficked in some pretty egregious conspiracy theories. But the biggest flaw I think is that it was a CDG with no mandatory events at all, for the simple and understandable reason that the designer didn't like them. Why should I, as a player, get a card in my hand that I don't want to play, a card that I'm forced to play for the event that hurts my position? It strips away my agency. At the very least let me play it for some Ops points.
Now, before we continue, I should put on my Official Game Design Weirdo hat, because I am someone who, improbably, has made a name for himself by designing sometimes aggressively weird and unconventional games. So far be it from me to chastise another designer, or his or her design, for failing to play by the rules. (Particularly because my own adventures in that space, This Guilty Land, is a pretty idiosyncratic departure from the predominant "Ops/Events", fog-of-war model.) It is entirely possibly that one could design a great and interesting CDG that doesn't utilize mandatory events. I for one would be interested in playing such a game, not because of any dislike of mandatory events on my part, but because for a CDG to work without them, it would need to be a radically different take on the genre.
The flawed CDG game I was talking about was not radically different; it hewed pretty close to the orthodoxy, except it removed the thing that makes the whole model work. It's like macaroni salad without celery: the whole thing becomes bland and mushy and just doesn't have the right bite. Mandatory events don't limit a player's decision so much as they give those decisions weight and consequences. The scoring cards in Twilight Struggle are a good example of this. You have to play that card, and you have to do it by the end of this turn, but exactly when you do it is going to make a difference. You want to play it at a time that will either maximize your scoring or minimize your opponent's, and you have to look at your hand of cards to determine what cards you should play, and in what sequence, to shift the game state where you need it to be, all the while being aware that you don't know what your opponent has in their hand, and that if you're too obvious about what you're on about, they'll cotton on to it and start playing their cards accordingly.
In This Guilty Land, the closest thing the game has to a "mandatory event" is the Violence cards. When you play one of these cards it dramatically reduces your support in every Region. It's not so much that you're explicitly required to play them: you can ignore them for the entirety of the game if you like. But they can't be discarded outright in the way that other cards can, and so they take up valuable space on your side of the Events Display, which in turn is going to limit the number of cards that will be drawn for you on the next go-around. Leave it there too long and you'll find yourself at a grievous, systemic, and perhaps irreversible disadvantage. Play it at the wrong time of course and you'll create a weakness that you're just begging your opponent to exploit. And, of course, there's never exactly a right time to play it.
Mandatory events can create a liability for the players who draw them - this is especially the case with events that only and clearly favor one's opponent - and in a way the games are about managing that liability. Players who manage it well do well, and players who don't, don't: it is, in some ways, the primary way in which the game rewards skill and punishes bad play. Without this liability to manage, the games devolve into a lot of tit-for-tat take-that, becoming aggressively and monotonously tactical, and more dependent on the luck of the draw. Mandatory events therefore give the players more agency and more decisions (and richer ones), not less.