From all psychology signaling is most powerful when it shows expensive resources spent on unpractical, useless things. As the peacock tail.

Spending a hundred bucks on something useful is not a signal showing wealth; but lighting a cigar with a hundred dollar bill is such a signal.

Being seen with, say, an expensive computer is less powerful wealth signal than being seen with an exactly equally expensive ring; the computer might be (even if it isn't) practical, but the ring states "hey, I can afford to waste money, so if you don't then obviously I'm of a higher social status/pecking order than you".

It's better if everyone in society can agree on the same signal to use. Otherwise it's too hard to verify.

It is more than that. Not only is it expensive, but its also small enough that you wear it everywhere. Think about why ipod/iphones are so popular amongst teenagers