It's a band-aid for not having the flexibility to adjust the suspension geometry the right way. The real way to do this is to get the correct size (wider) tire for the wider front wheels. Anyway, I think it's dumb because if you're going through the effort of getting more front grip, why not match it with more grip on the back? And I dare anyone to say "friction!".. Save it, this isn't a drag setup.

I can see taking away some grip in the back of an already understeering car, but unless there are class restrictions on the type of suspension-adjustment parts you can have, this is pointless. I think this may explain the autocross connection?
Keep the wheel widths the same all the way around and change more important things like castor, toe, sway bars, weight distribution, etc. Until I see the FWD BTCC cars running a setup like this, I really doubt it's merits.