It is dual purpose. A means of earning a living while providing shelter.
Anyway to continue the argument:
My take on all this is that if a fox finds a target/s while foraging it will take it, voles, rabbits, chickens lambs etc. I do not think there is a dispute on this. The dispute is what threat this presents to livestock. If I understand correctly, from the little rushed research I have done on this, losses of lambs due to dog/fox predation are low. I think this needs a degree of qualification. I would argue the losses can vary regionally and year to year and probably has a related weather dependancy.A hard winter will diminish other food sources for the fox and weather has a debilitating effect on lambs, making them an easier target. A ewe will endeavour to protect a lamb, but as singles become twins or triplets the job becomes more challenging with less likelihood of protecting all. I think it is a field requiring a lot more study to determine if a fox presence is beneficial or detrimental to the ecosystem it inhabits In conclusion attacks are rarely seen and what may appear to be the aftermath of an attack could be merely scavenging of a carcase.
http://www.discoverwildlife.com/british-wildlife/do-we-really-need-control-foxes-uk

Now seconds out. Round 2