I recently got a Luna Display ↗︎ for a reasonable price, and I really like it. This small review highlights the pros and cons, so you can decide if it is worth a shot.
Luna Display is a small USB-C (or HDMI or Mini display port) dongle you connect to your host computer (Windows/Mac), and in another Mac or iPad, you install their app, which enables the device as a second screen. Apple released Sidecar ↗︎, transforming an iPad into a second display wirelessly. This is good, but Sidecar has two limitations: 1) it only works with iPads as second displays, and 2) both devices need to be connected to the same iCloud account. The second reason blocks you from using a work computer with your iPad.
Luna fixes the limitations above. It can be helpful if you carry two laptops around, enabling one to be a second screen, or it can help you use an old iMac as a second screen while you can still use the computer if needed. Also, if you are using an iPad as a second screen, you can connect it to USB, not requiring you to use wifi while keeping your iPad device charged.