I don't really know, I would say that for the most part the quality isn't even particularly better, but I think you're paying for a better brand of high street name. For example, some people won't even shop in Primark, New Look etc so for them they'll just pay whatever price. For me, I can look at something and then I'll take some time to think about it and in the mean time will find something very similar somewhere cheaper and I'll buy it there instead because I'm incredibly fickle with clothes and get bored of them really quickly so there's no need for me to pay £40 if I can get virtually the same thing for £15. But if I didn't look in cheaper shops then I wouldn't know there was an alternative.
I think places like Warehouse, the quality really is better - a lot of their dresses are silk or a thick better quality material, but in some other shops like Topshop some of the clothing has material which is just as flimsy as you'd get in Primark.
I think it depends on your attitude to clothing. I like wearing the latest trends but I don't have a lot of money and I go for variety over having to wear the same item in ten different ways because I paid a lot for that one item and I then can't afford anything else. I like to be able to have a lot of choice when it comes to clothes so I buy cheaper but I buy more of them, whereas other people will use their money to buy one or two items from a more expensive store. Some things I will pay more for, like jeans, coats and shoes, but for everything else I personally don't think it's worth it.