The iPhone X, like the iPhone 8 and iPhone 8 Plus, features glass on both the front and back of the device. Among other things, this glass back enables the iPhone X's wireless charging capabilities. However, it's also, you know, glass, which is prone not only to shattering, but also scratching and other light damage over time. Replacing that glass can also get expensive, especially if you break the glass on the back of the phone. If you don't have AppleCare+, such a repair could run you as much as $549.
Luckily, there are options out there for protecting the back of your iPhone X that don't add a lot of weight or bulk. Additionally, a couple of these options offer not only protection but a degree of customization for your brand new phone.