Google Maps is already the most popular free app in the App Store, and it could be the heart of Google's efforts to stay firmly planted within iOS -- whether Apple likes it or not.

Jared Newman / TIME.com

As my colleague Harry McCracken reported last night, Google has come to the rescue of lost iPhone users with its own Google Maps app.

Although the app has been rumored for some time now, one of the big questions was whether Apple would actually approve it for the App Store. Harry says he was never worried; Apple doesn’t decline apps for competitive reasons any more, and the arrival of Google Maps just makes the iPhone a better competitor to Android. No problem, right?

Well, not exactly. Google Maps does make the iPhone better, but it also poses a threat to Apple on its very own platform. With a native Maps app, Google is starting to sink its hooks deeper into iOS. The app itself is just the beginning.

I thought TechCrunch’s interview with Daniel Graf, director of Google Maps for mobile, was an interesting read — particularly this exchange:

TechCrunch: How does Google feel about Apple now allowing another Maps app to be the default and Safari clicks not opening up Google Maps? [Note: I think this question was supposed to say “not allowing,” as is the case on iOS.] Daniel Graf: We want our native Chrome and Search apps to give you the opportunity to jump into Google Maps. With our new SDK, this is up to the developers to integrate into their apps if they like. Safari is out of our control.

(MORE: Problem Resolved: Google Maps for iPhone Is Here, Looks Good)

Apple doesn’t allow the user to decide which apps perform certain functions by default. For instance, if you tap on a link in Mail, it always opens in Safari, even if you prefer to use Chrome or another alternative browser. Likewise, if you try to get directions to a restaurant through Yelp’s app, you’ll be taken directly into Apple Maps, even if you prefer to use Google Maps instead. (For what it’s worth, Android allows users to select default apps for certain functions.)

Google is giving app makers an alternative. It will offer software tools (the “SDK,” or software development kit in techspeak, that Graf mentions) to send the user directly to Google Maps instead of Apple Maps for directions.

As for Google’s own apps, the Maps integration is rudimentary at the moment. If you use Google Search or Chrome to look up a location, it shows you a map. Tapping on that map takes you to the web version of Google Maps, where you can finally tap a button to jump into the native Google Maps app. In the future, I could see Google cleaning up this whole transaction and sending users straight to the native app.

(MORE: Why Google’s (and Apple’s Alleged) 3D Maps Don’t Seem That Exciting)

Google has been working to tie together its other iOS apps as well. Right now, when you view a link in Gmail for iOS, you have the option to open it in Chrome as well as Safari. And when you want to share a link in Chrome, you can send it directly to the Gmail app or to the Google+ app. Essentially, Google is building a platform within a platform — one where all of its services are connected, and working around Apple’s limitations on default apps by having Google-built apps talk directly to one another.

Google Maps is the centerpiece, because it represents a big source of revenue potential for Google. “The fact that a phone has a location is really helpful for monetization,” Google CEO Larry Page told Fortune just a few days ago. He was talking about Android, but the same is true on the iPhone. For tech titans like Apple and Google, location is a key battleground. Controlling the data means having the chance to offer new location-based services (or in the case of Google, to serve up location-based ads).

Apple reportedly expelled Google’s mapping data in iOS 6 because it wanted that control for itself. But because of the problems with Apple’s own Maps app, Google gets to waltz right back in. Now, it’s the most popular free app in the App Store, and it could be the heart of Google’s efforts to stay firmly planted within iOS — whether Apple likes it or not.