You may have noticed many smartphone cases have a large circle on the back, usually also with a vertical line. Those are magnets to improve compatibility with Apple’s MagSafe technology.

MagSafe was first introduced in 2020, alongside the iPhone 12 series—don’t get it confused for Apple’s laptop charging port with the same name. It allows accessories to snap to a phone using magnets, and the phone can even be charged through MagSafe using a modified version of Qi wireless charging. More recently, MagSafe was reworked as the Qi 2 charging standard, and may eventually show up in more types of phones.

MagSafe uses a large circular magnet array for the secure connection, with one additional magnet below the main ring that helps align accessories. Many iPhone cases have a similar ring to help MagSafe work through the case. Without those magnets in the case, accessories like MagSafe wallets and chargers would slide off the back of the phone. The iPhone’s own magnets aren’t strong enough for a secure connection with a case in the way.

Cases with MagSafe aren’t limited to just iPhones, though. Moment, Spigen, and other companies sell cases for Android phones with the same magnetic rings, allowing more phones to use Apple’s ecosystem of magnetically-attaching accessories. That’s why you might also see some cases with a ring that aren’t even made for iPhones.

Apple said in its 2020 announcement, “MagSafe delivers a unique experience to iPhone, featuring an array of magnets around the wireless charging coil, optimized for alignment and efficiency, that perfectly connects to iPhone every time. MagSafe chargers efficiently provide up to 15W of power, while still accommodating existing Qi-enabled devices.”

The magnetic rings might make some cases look a bit less cool, but the alternative is breaking MagSafe, which isn’t great either. There are still many third-party cases without the magnets if you really don’t care about MagSafe compatibility.