VTI captures the entire U.S. equity market, from large caps down to small caps, in a single ETF. At a 0.03% expense ratio it's extremely cheap, and its holdings count far exceeds the S&P 500's, but its top holdings are still megacap tech names like Nvidia, Apple, and Microsoft — so overlap with VOO or SPY is substantial.

It suits long-term dollar-cost-averaging investors who want "all of the U.S. market" in one fund. If you already hold VOO, there's little need to add VTI on top — it makes more sense as an alternative when you specifically want broader mid- and small-cap exposure.