The only reason to buy a new car, in my opinion, is if you drive in it every day for over a couple of hours, whereupon having mod cons actually improves your quality of life. Although IMO you might be better off buying an older, but better car second hand still, for less :-)
Of course most people will do it to keep up appearances with the neighbours. I.e., vanity.
My car is the 15 year old dented Honda Civic, and I earn in the top 10 percentile (or better, I just checked the government stats). But that's London, I drive it maybe 15 minutes a day on average, I have to park on the street (hence the dents, other drivers suck), it cost me about £2300 five years ago (cash) and I guess I pay £1200 a year to run it (insurance, tax, MOT, petrol). Even if I give it away, that's under £1700 a year for freedom and time saving ability (if I didn't have a child to drop off at nursery, etc, I could drop the car and just hire a vehicle for when I really need it).