Until now, iPhone and iPad users could not delete Apple’s built in apps, so many resorted to stashing them out of sight in a folder labeled “junk” or worse. But in iOS 10, which became available on September 13, users can delete 23 of the Apple apps, as was reported months ago.

But some of the apps that can be deleted provide features or functions to other parts of the iOS operating system. And when they’re deleted, sometimes the functions can still be accessed, but sometimes they cannot. Apple says deleting the apps saves only about 150 MB of space. Deleting an app generally also removes associated data and configuration files, but not always.

For example, deleting the Contacts app doesn’t delete the information about all the contacts. That info can still be used in the Phone app. Deleting the FaceTime video conferencing app doesn’t remove the same capability from the Phone app—or the ability to initiate FaceTime calls from the Contacts app, if that app is still around.

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But deleting some apps removes functionality from other areas. Get rid of the Music app, and some audio content in Apple apps and other apps won’t play on some car stereos of stereo receivers, Apple (AAPL) warns. Remove the Weather app, and there will also be no weather forecast in the iPhone’s notification center or on the glances of a linked Apple Watch.

Deleting the Maps apps did not appear to remove mapping functionality from third party apps that rely on Apple for the feature, such as Yelp.

Finally, users can bring back any default app that they’ve deleted using the App Store app, which cannot be deleted. Simply search for the missing app in the App Store and click the download icon (the cloud with an arrow pointing down) to bring back.

The iOS 10 software will be pre-installed on the new iPhone 7, which hits the market on September 16. But Apple made the software available for download a few days earlier for owners of current iPhones and iPads.