I don't think buying stocks helps the economy. For you to buy them someone else has to sell them. Until something tangible is created all this trading is just a game of symbols. Stock owners like us are all rent-seekers. We seek to seize control of an income stream produced by other people's efforts instead of working ourselves. Our capture of the rights to corporate income does not produce widgets or widget services - the workers do.
The liquidity provided to the market by our buying activity helps the economy.
When you buy the stock of a company, thereby potentially increasing the value of the stock, you potentially help pay for the RSU/ESPP part of the salary of a worker.
But, biggest of all, the presence of a liquid market where *anyone* can become an owner itself has many societal benefits.
Because of the above I don't quite buy the assertion that to RE is to become a rentier.
(There is, of course, the additional factor that you add value to the economy as a consumer and also - hopefully - intangible value as a community member.)
I think the question is how does "liquidity" "help the economy?" Certainly a lack of liquidity would prevent the formation of new firms, hamstring their expansion, cut off investment in new products and tech, etc. At liquidity = 0 we would be living in a highly inefficient barter economy. Firms obtain cash through share offerings and debt offerings. So yes, if you buy into an IPO or debt offering, you are handing over your money for productive use.
However, the percentage of stock trades that meet that definition is probably less than 0.0001%. What is actually happening on a daily basis is people are buying and selling ownership tokens amongst themselves. To buy stock in the market and say I provided liquidity for a company is like buying a 30 year old house and saying I built a house. No, I traded cash for a pre-existing asset. The builders got nothing from me. It's also a stretch to be a day trader and say "I provided liquidity for a company" as if day trading a stock were some kind of banking service beneficial to the firm whose stock was swapped amongst parties unrelated to the firm.
Of course, any asset would have a much lower price if it could never be sold in a secondary market. Imagine buying a house or a car, and having to sign a contract that it could never ever be sold. You'd pay a bit less for the asset due to that loss of optionality. Thus being sell-able adds value to an asset.
The most persuasive (and conventional) argument is that by participating in a marketplace, we inflate the value of assets by making them more liquid. This benefits firms by lowering interest rates and inflating asset prices. Thus they can obtain capital at lower cost. Thus they can invest in projects with lower expected margins. Thus more projects are invested in, and economic activity increases.
I've read that in the era before securitization, illiquid loans to creditworthy borrowers typically ran 10-15%. That's a long way from the 2-4% yields we see on today's IG corporate debt, and the 4% earnings yield on stocks at today's multiples. Fast-growing liquidity has raised asset prices to the point that investment returns are arguably too low to support retirements. This too may be good for the economy. If we all must work until we die, and are thereby blocked from being rentiers, more was probably produced. But it also exposes the folly of this whole approach. If perfect liquidity means a near-zero cost of capital, such an economy would be indistinguishable from serfdom, because there would be no way for a worker to advance to the rentier class through their savings. Even worse, low returns create incentives for workers to save more (see the savings rates in China and Japan). High savings reduce consumption, which kills the economy in the end.