The tax case rests on a single structural fact, and it is easy to state precisely. Inside a fund, the gains and losses on individual holdings are the fund's, not yours. They net against each other within the portfolio and reach you only as a distribution or as a change in the value of your one position. A shareholder in a fund that rose 9% over a year has one holding with one unrealized gain, and no loss to take, even though many of the companies inside the fund fell during that year.
Holding the constituents directly changes what is available. Each position is a separate tax lot in your account, so a holding that has fallen can be sold at a loss whatever the index did, and the proceeds reinvested to maintain the exposure. What the resulting loss is then good for, and the wash sale rule that governs how it is done, is the subject of tax-loss harvesting rather than of this page. The point specific to direct indexing is narrower and is the reason the approach exists: the losses are reachable at all, because the positions are yours rather than the fund's.
Two limits on that argument deserve equal billing. Realizing a loss lowers the basis of what you buy back, so the benefit is largely a deferral rather than an elimination, exactly as it is with any harvesting. And FINRA points out a cost specific to selling individual holdings: "a stock you sell might soon rally back above the cost basis; however, you'll have lost that potential gain as well as altered your portfolio allocation."
The second use is exclusion, and it is more compelling for a narrower group. Because the holdings are individually owned, a company can simply be left out and the remaining weights adjusted. The most defensible reason to do this is not preference but exposure. Someone whose salary, bonus, job security and equity compensation all depend on one employer already has a large economic position in that company, and holding it again inside every broad index fund they own compounds a risk they did not choose. Excluding it is a way of reducing a concentration rather than of expressing a view. Screens applied for other reasons are equally possible and are a matter for the investor.
What the approach costs, and FINRA is direct about both parts. On tracking, it warns that by deviating from the index, "your returns may be different from the index's returns and could potentially be markedly lower." That is the unavoidable consequence of holding something that is not the index: every exclusion, every harvesting sale and every reinvestment introduces a difference, and differences run in both directions. On cost, it warns that "you might also incur higher fees following a direct indexing strategy than you would with a typical passive portfolio," which is worth measuring against the value of the losses being harvested rather than assumed to be smaller than it.
Three practical consequences follow that are easy to underestimate. Holding hundreds of positions means hundreds of tax lots to track, and every corporate action, merger and index change touches the account. The approach does nothing at all inside an IRA or a workplace retirement plan, because losses there are not reportable, so it belongs in a taxable account or nowhere. And the benefit is largest for someone with substantial gains elsewhere to offset and a high rate applying to them, which means its value varies enormously between two investors holding the same portfolio.