Actually, there are many reasons and points to note here:

1. The clap thing was not new; Medium had been experimenting with it for a while now, albeit in Series and not within the stories.
2. The point was how does a reader show his appreciation for a story he liked? How would he demarcate between two stories — one that he liked and one that he looooved.
3. One could argue that the same could have been achieved by giving more than 1 like or recommend to a story. The problem is, that’s not how we are used to like or recommend in life. With claps, it’s more aligned to our experience. Think of giving 50 claps as the same thing as giving a standing ovation to a maestro vs a couple of customary claps to an average performance.
4. The biggest, and maybe the MOST IMPORTANT, reason — Medium and Ev Williams had been talking about getting Medium behind a paywall where users would need to pay for quality content, and writers would be able to monetize their stories. Having a graded appreciation system helps them refine that process. Users pay $5/mo to Medium, but how will that be distributed to the writers? On what basis should their payouts be calculated. Sure, number of views is one variable to consider, another would be number of hearts/recommends/likes. But if the rate of hearts (claps) per appreciating reader is high, shouldn’t that also matter? It also gives Medium a more intelligent way of deciding which stories to promote more.

I may be off on a number of points here, but if you think about it, there is merit in this move. Sure, it is a bet. After all, all experimental moves are.