Bitfinex is the largest BTC exchange in the world, over $500 million per day in volume today. Big days are multi-billion dollar volume. Like most exchanges, they take a percentage cut of every transaction. This is not a company with zero value. The "buy low sell high" arbitrage thing is from a previous, unrelated business idea posted 5 years ago to a btc forum by the founder of bitfinex. As far as I know, it never went anywhere.