Winning players go broke. Not often, and not because they were wrong about being winners, but because a real edge is small next to the noise it arrives in.
Enter your win rate and standard deviation — both per 100 hands, as every tracker reports them — and a bankroll. It gives the chance that bankroll never recovers, and the range a stretch of hands is likely to land in.
A 3bb/100 win rate against an 80bb/100 standard deviation means the noise is roughly twenty-six times the signal over any single hundred hands. Over a long enough run the edge wins; over a short enough one, anything happens, and the only thing standing between the two is the size of the bankroll.
Most players can quote their win rate and have no idea of their standard deviation, which matters more. Tight no-limit is around 70–80bb/100; loose games and pot-limit Omaha run well over 100. Doubling it quadruples the bankroll you need for the same risk.
At these numbers, a losing stretch of 25,000 hands is entirely ordinary for a winning player. That is why the tool shows a range rather than a figure: the average is the one outcome you will almost never actually experience.
For a solid win rate and normal variance, 20–30 buy-ins keeps the risk of ruin near 5%. For a thin edge, considerably more.
The chance that you lose the entire bankroll before your edge has time to show, assuming you keep playing at the same level.
Enormously. It cuts the swing in cash terms while leaving the edge intact, which is why it is the standard answer to a bad run.