So this extension functionally works as if your single tank just got larger. Might not be something that everyone cares about, but the value proposition over a jerrycan is clear.
1. Generally you don't need the excess range, and this thing seems like a relatively permanent modification, so you're dealing with the excess weight/bulk all the time, and getting the added benefit only when you actually need to do the long haul.
2. This Ducati thing is only 8 liters, and adds 40% to your range.
So a jerrycan would (more) easily be added/removed as needed, and holds 20 liters, doubling your range. You could even add two, to triple your range.
Also, there's the fact that a jerrycan costs about $40, while the Ducati tank costs... $1900!? Yikes -- yeah, I'm sticking with the jerrycan :-)
But I get it -- I've ridden with Gatorade bottles full of spare gasoline too (yes it'll eventually eat through the plastic, but not fast enough to matter in my experience.)
Samesies! I once ran out of gas on a GS-550, walked to a Circle-K, bought and drank a quart of milk (I'm not much for gatorade) then bought enough gas to lightly rinse out the paper carton and then fill it to take back to my bike. This was a long time ago, please don't judge me :-)
https://www.ducati.com/ww/en/accessories/ACC012304
Something like 350 mile range (lot of variables) once that's added on. Enough to get from Coldfoot to Prudhoe Bay (one of the harder routes to clear on a bike, historically).