As an analogy, imagine that your home is wrecked in a hurricane, but a scraps dealer is able to offer money for what's left. If there's only one scraps dealer around, they will absolutely rip you off and pay you just enough that you'd be livid and kicking but still aren't motivated to find a way to sell the scraps yourself. But if there are two scraps dealers competing for the wreckage, it'd be easier for you to get a market deal.
Scraps dealers improve their profitability by finding better ways to sell the scraps, so in a competitive environment, you could indirectly benefit from their experience. You make them sound like terrible people by saying they "make money off a disaster", but objectively their existence is a net benefit to the situation. One thing that can make this argument break down is that scraps companies may rely a lot on information asymmetry and price gouging, but that is actually less of problem for financial transactions that can be systematically electronified, where there's a lot of regulation and said regulation is more easily enforced.
Making markets efficient is a very abstract concept that people makes people feel disconnected from the benefits. But in this abstract way, it does contribute to the economy.
Not enough smart people thinking like Jane Street (or any large trading company with a track record of managing risk), but still performing financial activities, is arguably what causes financial crises to begin with. Think people buying into schemes based on returns that are too good to be true, bundling bad mortgages without modeling it properly, or banks not taking clients' money seriously.