This thread is def at risk to blow up lol but to answer your question Ree a big part of hip hop/rap culture (which is predominantly a black subculture) has been showing off all the materials/resources you have in comparison to other competing males to show dominance and success as a man. Thus showing off materials/resources became vogue as a means of publicly displaying their status.
Definitely an aspect of attention seeking which could be a byproduct of growing up in a rough situation causing low self esteem and a desire to be noticed/appreciated.
I was always interested in black inner city and hiphop/rap culture growing up and had a lot of friends immersed in it and its typically the "hot boy" cocky guys that engage in this type of behavior. Plenty of black ppl ingrained in rap/hiphop innercity culture that are very humble and maybe only wear a modest chain or rims on their car.
Money = status in black innercity of the US so by flashing how much money you've gathered you thereby indicate how high status you are (status getting you more respect, women, etc.)