Indexes are tradable through etfs, futures-contracts (es, nq), and options. Because most funds are measured against them, index composition changes force real buying and selling, especially at the close (market-on-close).
Market-cap weighting means a few large companies dominate: the top 10 S&P 500 stocks can be over 30% of the index.
Example: a stock with a 7% weight in the S&P 500 rises 5% while everything else is flat. The index rises about 0.35% on that one name.
Related: etf, es, market-cap, market-on-close