The "Why Register?" screen is available from the Help menu in the free version of RJL System Cleaner. It shows you a clear summary of what registration includes and why it is valuable.
If you have run a scan in the current session and duplicates were found, the screen displays your personal duplicate file count and wasted space at the top. For example: "Your last scan found 47.2 GB of duplicate files across 847 files."
Below that, it lists every registered feature with a description of what it does and why you might want it.
At the bottom, it shows a "Register Now" button that opens the registration page in your browser, and a "Maybe Later" button that closes the screen.
The screen is purely informational -- it does not pop up on its own, it does not nag, and it does not block you from using the app. You open it only when you are curious about what registration offers.