The way I see it, it's the ultimate evolution of Poch's preference for flexibility. Playing Foyth as a nominal RB and Sessegnon as a marauding left-back with limited (if any) defensive responsibilities allows for all sorts of very, very appealing tactical combinations.
Foyth shifts left and forms a back three with Sanchez and Verts, Sissoko ahead = solidity. Foyth and Verts can pass out from the back with ease, Sissoko and Sanchez provide pace and strength.
If we need more men forward, Sissoko can go up, the back three remains - still relatively secure, while giving the opposition another headache.
If even more men are needed, Foyth and Verts can maraud down the left and right half-spaces (both are good on the ball), Sissoko and Sanchez can stay back (pace can prevent breakaways).
And all the while, with Son on one flank, Sessegnon on the other, and a middle four of Ndombele, Lo Celso, Alli and Kane, we're creating an incredibly dangerous side down the middle that can't be easily dispossessed and can press for days - and can also seamlessly switch positions if needed.
We also have the personnel to essentially create a mirrored copy of this same system with a right-facing emphasis - Aurier/KWP becomes the wing-back on the right, Davies becomes the nominal centre-back from the left, and everything builds from there. So even if the opposition somehow deal with the system, two substitutions will completely shift it to a right-sided emphasis, and create a new problem.
The more I think about it, the more advantages this system has. It's supremely flexible, and all it needs is a couple of intelligent players on the flanks. Like Sessegnon.