An s-corp can indeed use the 20% Qualified Business Income Deduction that was part of the TCJA legislation. But even without that, there is a much different, much more important benefit to an s-corp.
It is closely related to the idea that shareholders working for the company must get reasonable compensation, meaning that they must get a salary or wages on a W-2 that realistically reflects the work they are actually doing. Say a doctor operates his medical practice as the sole shareholder of an s-corp, and he has net profit of, say, $1 million, after all other expenses. He can't take a W-2 salary of $24,000 per year. The world doesn't work that way. He's not a front desk clerk at Motel 6. His salary has to be equivalent to whatever it would be if he was practicing medicine as a real employee and not a business owner. So that reasonable salary, for that doctor, might be, for example, $400,000 per year. It all depends on variables like what his specialty is, and what part of the country he's in. But the point is that if you do it right, in this example, the other $600,000 of profit does not go on the W-2, but rather on the K-1 as investment income. So that portion of the profit is not subject to social security, medicare, or self-employment taxes.
This is the main reason for using an s-corp. In this example, the outcome is much better than having $1 million of net profit on Schedule C.
And if it is done the right way, it can work for traders.
BMK