Diversification

Spreading money across many companies, sectors and countries makes you less dependent on how any one company does. If one share falls 50 %, that is only 5 % of a portfolio of ten equal positions.

Diversification reduces single-stock risk, not overall market risk: in a crash almost all shares fall. A broad ETF is the simplest form of diversification; individual purchases after dips tend to complement it rather than replace it.