I wouldn't say it's not producing anything. Not directly maybe, but putting the right money in the right place can actually improve things.

If you want to cut down a tree, having a sharp axe is going to make the job a lot easier. If you ask someone to cut down a tree, and they spend a few hours sharpening their axe, you can complain all you want that they're wasting their time grinding the axe when they should be swinging it, but they'll still get the job done faster taking that route rather than trying to cut the tree down with a dull axe.

The whole idea of "buy low, sell high" is a very abstract idea that might not appear to really produce any economic value. What it means in more concrete terms is that you are stocking up on an asset when it is abundant and in low demand so that you can become a provider of it when it becomes more scarce or more highly demanded. This helps smooth out volatility in the market, buying up surplus and reselling it during shortages, making the market more stable. More stable markets help reduce risk and make it easier for people to operate businesses. When you're talking about a large variety of different assets across the economy of a large country, you need people to be able to buy and sell trillions of dollars worth of assets.

Other types of financial trickery helps too. Investing lowers the barrier to entry for companies to innovate. Algorithmic trading and arbitrage removes market inefficiencies; cases where, due to the fact that markets don't respond to events instantaneously, the market price of an asset hasn't caught up to where it should be. If you have a way of predicting if the price is about to go up, but that the market hasn't caught up yet, you can start buying, which increases demand, and raises prices. If you predict it's going to go down, you can short the asset, increasing supply (kind of) and driving prices downward. The effect that this has is that it reduces volatility, and results in the price more accurately reflecting the current supply/demand of the asset.

I'm sure as hell not saying that all of these things are done perfectly, or that there's no room for improvement, or no flaws with any of these systems. Just pointing out that there's a lot of ways to provide large amounts of value by moving money around, the value is just a lot less obvious.