It is clearly best for the entrepreneur to get as much money in the door as possible, to maximize runway. But if you're having trouble closing the amount of money you want, there's no harm in setting up a trenched deal (i.e. triggers).
The advantage for the investor is that they are putting money in at series A prices, but a portion of it is at a lower than series A risk level. It's a better deal for them.
The advantage for the entrepreneur is that unless everything blows up you're pretty much guaranteed to get the money. If you get 24.5K registered users you have some real traction. I'm sure the investor will be thrilled to put in the money at series A pricing. They really have to anyway, they won't walk away from their investment.
If things aren't going well, they can walk away. But it would only happen in practice in extreme cases, craziness like Google coming out with a product that obsoletes yours. (And they'll still walk away even if you hit your target if the business really does stop making sense).