I have one for my Frame and it’s excellent!
The built in cameras are indeed only there to do tracking and looking through them is pretty janky - lots of distortion and the frame rate is poor which can give people motion sickness.
Using the Arcturus, you’re able to keep the Frame on while checking your phone etc. For instance my dog needed out and I could just put on passthru and let him out without needing to remove the headset. Another example, I was trying to set up SteamVR and get Elite Dangerous streaming from my PC. With the Arcturus, I was able to see my computer screen via passthru vs taking the Frame on and off repeatedly.
It’s 100% not a must-have accessory but having the Arcturus’ clear, color, high frame rate passthru is a much better experience and I won’t be taking it off the Frame.