Risk of ruin
Risk of ruin is the probability that a losing streak shrinks your capital to the point where you can no longer trade, or below a threshold you set.
It depends on three inputs: your win rate, your win/loss ratio and the percentage of capital risked per trade. Even a strategy with positive expectancy can wipe out an account if the risk per trade is too high.
Losing streaks run longer than people imagine. With a 50% win rate, seven or eight losses in a row will eventually happen over a few hundred trades.
This is the strongest argument for low risk per trade: cutting your risk from 3% to 1% doesn't just divide your loss per trade by three, it makes ruin far less likely.