Because if the state didn't provide those payments, the children would not be eating, wouldn't live in safe housing, wouldn't even be safe. That's why.
All these posts about "you have more than I do" - she has 7 children, you probably don't. £4k doesn't go far with 7 children. The benefits will stop when her children reach 16, or 18.
I actually think she is doing ok - she has got an eye on the future - most people who are reliant on benefits, end up with no pensions, living in rented accommodation (if they can still afford it once the kids leave home). You won't be in that situation.