A software engineer might not need to impress anyone to advance their career. But I think some careers do require a person to first look successful in order to be successful (sales(real estate), management, law, many kinds of consulting). I don't know how much someone has to spend on appearance in those situations but maybe the people themselves don't know either and so over-spend just to be sure. Maybe the $1000 jeans would help your friend impress the truly wealthy and maybe they wouldn't.

Which isn't saying that some people don't compulsively and unnecessarily spend. But in mid-level careers, determining how much is over-spending may be hard (which increases the temptation to spend, naturally).