Unfortunately, what this API currently does is pretty primitive: It accepts a prescription number and then reports back to the app that it has (or hasn't) successfully requested a refill. Just the fact that there's an API is a big step forward, because it means Walgreens can extend that API without breaking any apps that use it.
For example, using the API, Walgreens could cleanly add the ability for apps to get more useful information, such as: an estimated time the refill will be ready; the price; whether there's a brand name that could be substituted if the pharmacy is out of the generic; or whether there's another nearby store that could refill it sooner if the original store is currently out of the medication.
Those are all things the chain could easily provide in the future, and it doesn't need an API to provide them. The advantage to an API is that the chain doesn't have to commit to those features now. That not-especially-interesting architecture could make some much more interesting features available going forward, and with a minimum of extra effort on Walgreens' end.
An API like this isn't remotely a new idea—which is actually an advantage, because no drug chain should be risking the use of yet another untried technical architecture in the face of endless privacy and regulatory concerns. There's already too much shakiness in the link between smartphones and corporate pharmacy systems.
And there's a real risk for Walgreens in setting up an API. The chain is requiring developers to register with the chain before it can use the API, and the apps will have to pass authentication information to do even a simple refill via prescription number. But, at a certain point, it will become possible for third-party developers to create apps that do the same thing with similar APIs from Walgreens' competitors, offering the prescription equivalent of a mobile wallet. For the moment, Walgreens is alone.
If a customer shows that list of prescriptions to a competing pharmacist to make sure there are no problematic drug interactions, might that pharmacy try to steal the customer for the prescription Walgreens currently fills? Maybe. On the other hand, it's probably a lot safer for any customer who is making trips to multiple pharmacies to bargain hunt.
At the risk of sounding snide, we're glad to see a drug chain's app whose architecture has finally made it all the way to the 1990s. Now, we're just waiting for the rest of the features to catch up.