For animal husbandry barbed wire is completely unnecessary. They don't climb fences. No, barbed wire has one purpose and one purpose only, and that is to deter humans.
1 strand of barbed wire is the norm, for a good stock fence as the top strand for the reason pointed out by Ninethace. Other strands are only necessary if the livestock are likely to push their heads through a gap. But there are plenty of signs in fencing that are pointedly anti-visitor.
Barbed wire wrapped on a top rail of a locked 5 bar gate, almost certainly human deterrent. One rarely sees a gap near the hanging end of the gate, which might indicate an understanding that countryside visitors might be using the gateway.
A pile of rusty barbed wire loosely discarded on the outerside of a fence is another indicator.
I once visited a disputed path on the morning of the Public Inquiry. There was another person inspecting it and the fence that was in position, where a gate should be. I help him over the fence, top strand barbed, then plain high tensile and pig netting, fairly standard all purpose fence. However it was booby trapped, the staple that should have been the middle staple for the netting was one stand lower. This means that when someone climbs over, the netting will swing out when they are astride the fence and risk tearing trousers or even castration.
Something made me hold back pointing this out, it was just as well as that person was the Inspector for the Public Inquiry. If I had voiced my suspicions I might have jeopardized his impartiality for the inquiry, as it was he bought our chance meeting to the attention of those attending.
The PI was successful in favour of the route being a new or recovered Right of Way.