The biggest feature of Apple Pay could be something no one's talking about

Apple Pay is finally coming to the UK in July, allowing people to pay at places like Starbucks and Boots by tapping their iPhone on a card reader.

But people who are working with the company on Apple Pay aren't excited about that.

What they are excited about is the forgotten other feature of Apple Pay: in-app purchases.

As well as allowing people to pay for things in-store with their phones, Apple Pay lets people buy things on mobile and the web with the click of just one button.

Sign up to Apple Pay on your iPhone or iPad and input your card details, then go to any website or app that supports Apple Pay, click the button and you've made a purchase. Apple knows who to charge because of the particular iPhone used. Card details are tied to the Apple device, much the same way card numbers are linked to iTunes and App Store accounts.

John Collison, one of the co-founders of $3.5 billion (£2.25 billion) payment processing startup Stripe, says it's this feature that's getting businesses most excited, not the contactless mobile payments.

In his speech at MoneyConf in Belfast this week he said: "We do mostly the in-app [Apple Pay] and we're seeing a lot of growth. In the US there are a lot of apps we see where Apple Pay is the priority for them. They're getting a significant chunk, not a majority yet, of their transactions from there."

John Lunn, senior global director for mobile payment company Braintree, which was bought by Paypal for $800 million (£512.18 million) in 2013, also thinks Apple Pay's in-app element is the most exciting thing about it.

He told Business Insider at MoneyConf: "Everybody's talking about the in-store stuff but actually when you look at the presentation when they launched it, the merchants that were sitting behind Tim Cook were online."

Uber, Groupon, Eventbrite, Houzz, Airbnb, Hotel Tonight, Instacart and OpenTable were all featured in the Apple slide. All operate online-only.

Lunn says: "There's possibly more traction online. You see big companies that are using Apple Pay online now. But the headlines are coming from in-store because it's such a new experience."

The reason retailers and industry figures are getting so excited about Apple Pay on mobile is because mobile commerce - people actually buying things on their smartphone - is terrible right now.

Collison said in his speech: "Mobile conversion sucks. You see the problem in the divergence between where people are spending their time online and where people are spending their money. People spend their time on their phones and in a few apps like Pinterest and Twitter. But you can't buy things where you spend the majority of your time."

Apple Pay could change that by taking the hassle out of buying on your mobile. Rather than having to type out your address and card details on a tiny phone screen, you just click the Apple Pay button and you're done.

Buying everything from clothes to power drills on your mobile could become as easy as ordering an Uber. And every retailer wants to be the next Uber - that's the real reason companies are getting on board with Apple Pay.