Full Kelly maximizes long-run bankroll growth — but the variance is brutal. A 55% bettor running full Kelly on -110 will see drawdowns of 30%+ regularly, even in winning years. That kind of swing is psychologically damaging and frequently leads bettors to abandon the system at the worst time.
Half Kelly sacrifices about 25% of long-run growth for roughly 50% lower variance. Quarter Kelly sacrifices about 44% of growth for 75% lower variance. For most bettors, half Kelly is the sweet spot — meaningful growth without bankroll-destroying swings.
Note: if your edge estimate is wrong (and it usually is, at least a bit), full Kelly can be catastrophic. Fractional Kelly provides a buffer against your own overestimation of edge.