Currently, Signal only lets you make a local backup, which you must manually transfer to your new device for the restoration process to work. With the new tool, you can move your entire account without much hassle. On top of that, the connection between your two phones is end-to-end encrypted for extra peace of mind.
Right now, the migration feature is only live in Signal beta v5.5.1, and both phones must run this version for the transfer to go through. It should be available for a wider audience once it graduates to stable. In the meantime, you can test it out by downloading the latest beta from APK Mirror or joining Signal's beta program from the Play Store widget below.