Stakes make a difference, because stakes keep the tension in a story as tight as you want it to be.
Ideally speaking, you want the story to start as the poker players (your characters) are about to start playing. And then, with every game, they increase their bets, increasing their stakes in each game (chapter). If you really want to get things tense, you can lock each player in and let someone else (the bad character, perhaps?) increase the bets for them.
The reason why I say this is ideal is because the reader gets to know the character before all hell breaks loose. So they know who the character is. Then as the stakes increase, we get to know them better. We learn to care about them and how they react to challenges. And then just as the reader gets to the middle and thinks the character can't take more, that final $50000 game starts. The life changer. The one that will ultimately change that character - for better or worse - forever. That's good reading.
Sometimes, though, the poker game is longer than others. For example in a series, there might be a few big rounds towards the end of each book. Rounds so big that the reader thinks that it's the life changer. But the real life changer will occur in the last book. Otherwise, why would the reader bother sitting through the stories after that?
So, if your middle is sagging, odds are that it's because none of your characters are making any bets. There's nothing happening to make the reader worry about what the character stands to lose. And that's a huge problem when your story is about to go towards the climax. After all, the climax is about where the character wins or loses the most.
Make sure that the reader can sense what's at stake. You don't need to spell it out. Just make it big enough to spot. Hint at the possible results of failure. And of success. And above all, give them a feeling of the odds.
And then for maximum tension: In the life changing round of the poker game that is your character's story, force them to go for the royal flush.
Look Out for These:
1) Middle sagging because you either put the stakes too high too early, or didn't raise the stakes.
2) Undefined stakes.
3) CPs and betas doubting why they should be caring.
What's your approach to stakes in a story?