I agree; it comes from the stock market.  In those cases it's relevant, since it gives someone an idea of how much money it would take to buy a company--if you're aware of all the LBOs back in the day, you realize how important that is.  For crypto I don't think it's anywhere near as important.  You might have a coin with an extremely large market cap, but who cares if no one is using it for anything besides speculation (which is the case with most coins).

Personally I don't pay attention to these sorts of rankings.  They're unimportant, not just to me but overall.  There isn't anyone who's going to buy up all the bitcoin in existence, and the same goes for most of the top-ranked coins.