This is a well made and flexible little unit. On face value being able to produce a microphone level out to an audio interface without using a microphone doesn't seem particularly exciting. If you already have a mic then you probably think this won't be much use to you. I think in the ideal world most of us would rather have the flexibility to swap mics and use different mic'ing positions but for many people mic'ing cabs throws up all sorts of problems with extraneous noise and poor room acoustics that can be an expensive problem to fix. If those issues are a problem for you then recording direct does make life easier.
In this configuration the Red Box receives the amp to speaker output, sends this on to the speaker cab via a through jack and using phantom power or a 9v battery sends an XLR balanced signal to your audio interface or to a mixer. With the addition of an attenuator this then facilitates recording valve amps when the rest of the family are in bed. Silent recording or practice.
My own use runs in two further configurations. First, I run the Red Box as a separate input whilst recording a track with a mic'ed cab. The purpose of this is twofold. It works as an insurance policy should the mic'ed signal pickup spillover and it also provides me with a reference signal when I am trying new mics and mic'ing positions.
Next, I use the Red Box as the feed for a Wet/Dry/Wet stereo effects setup. The Red Box sits in the speaker feed and sends it's signal to an fx processor that returns to the mixer and powered speakers or headphones. Long atmospheric reverbs or heavy repeating delays can be quite inspirational for the player but aren't great for recording because you can't undo a processed signal. In this setup though you can still mic up the dry speaker cab, record it into your DAW and non-destructively add fx later.
Lastly, and my favourite trick.... Got an amp you love that doesn't have reverb or an fx loop? Stick a Red Box on the output, run it into your pedalboard and back to a powered speaker situated next to the main cab and now that all original vintage amp doesn't need to be modified as it has it's own "post-output stage parallel fx". And even if that is just adding reverb onto a non-reverb amp it more than justifies the Red Box purchase price.