Your Plex setup can contain many different components - for instance, a Server, Desktop Apps, Mobile Apps, and more. Plex keeps track of these devices when they sign into your Plex Account. This is useful for several reasons.

Mobile Sync
Users with an active Plex Pass subscription get access to Mobile Sync. Mobile Sync lets you select media from a Library then Sync it to a device. This process copies a transcoded version of the media to the physical device meaning you can watch or listen to it even when you don't have an internet connection. The Devices you can sync to are stored in your Plex Account Devices list.

Related Page: Plex Pass > Mobile Sync & Cloud Sync

Local Device Discovery
Discovering devices on your local network can be a technically tricky thing to do. There's nothing more frustrating that turning on a Device and see that it has problems discovering other devices on your network. Plex solves this by using it's own discovery method, GDM (or G'Day Mate!) along with a map of local IP addresses kept in your Plex Account. Simply put, the Devices list makes discovery of devices on your local network more reliable.

Managing your Devices
A list of your Plex Account devices is available from the Plex Web App. You can remove devices if you want there.