1. Time - I quote buy and sell prices continuously in Microsoft stock trying to earn the spread between them. You want to buy stock and my offer to sell is the cheapest in the market. We trade. Later, someone else comes along to sell and I have the highest bid, so I buy the stock back from them.
In between I'm exposed to price fluctuations, so sometimes I make money and sometimes I lose, but I make a profit on average by charging a spread between my buy/sell prices and predicting small price movements. The two of you could have met had you waited, but I let you lock in a sure thing by bearing risks you didn't want. Think of this like insurance. Odds are you spend more on car insurance than you're expected to receive in payments, otherwise the insurers would go out of business. But you probably prefer spending $1000 a year vs. spending $0 most years and $30000 once in your life.
2. Place - You're in the US and want to buy Nokia stock. Someone in Finland wants to sell for a cheaper price. You don't have access to the European exchanges and she doesn't have access to US exchanges, but I do. I trade against both your orders and earn a couple cents per share in profit. I helped you meet halfway across the world and also keep prices efficient to reflect global supply and demand.
3. Product - You want to invest in an index ETF that holds 40 different stocks. I quote an offer to sell based on where they're trading in the market plus a small profit margin. You buy my offer. I turn around and buy the stocks to hedge my risk.
I'm better at trading than you are and have very low costs. Even considering my profit margin, it's cheaper and less risky for you to buy the ETF from me than 40 individual stocks. I keep the ETF and stock prices aligned and also helped people who wanted to sell stocks get their orders filled. Win win win.
I think the work I do clearly benefits the market and natural investors. All of these trades are competitive too, so I can only earn razor thin margins per trade, or someone else will undercut me.
I do sometimes question how the market picks winners and losers, typically based on speed. Is someone whose system is hundreds of nanoseconds faster more deserving of arbitrage profits? Do high technology costs naturally lead to consolidation and reduced competition? https://faculty.chicagobooth.edu/eric.budish/research/HFT-Fr... this paper has some interesting thoughts on the topic.
FWIW I do think continuous price-time priority trading is flawed in that it creates an arms race in speed, but it's the least bad option. Other mechanisms like batch auctions are even worse. As a thought experiment, consider the #3 ETF trade I described. If I can't immediately hedge my risk and instead go into a batch auction for each stock, how will that affect the price I offer in the ETF?
PS: I do think it's a fun job. You get to play with cool technology and it's like a game that gets harder every day against very skilled opponents. Even the arms race isn't all bad. HFT firms have effectively bankrolled modern innovations in high speed ethernet, FPGAs, low latency kernel bypass networking, etc. that have knock-on benefits for other applications.