Annual turnover of 100% means the whole book is traded once a year, 1,200% means monthly, and a daily strategy can exceed 25,000%. Multiply turnover by round-trip cost to get the annual drag: 1,200% turnover at 0.10% round-trip is 1.2% a year, which is survivable; 25,000% at the same cost is not.
Reducing turnover is often the highest-return improvement available. Entry bands with hysteresis, minimum holding periods, and trading only when the target position differs from the current one by more than a threshold can cut turnover by half while barely changing gross returns.
When comparing two strategies, compare net of their own turnover cost, not gross. The higher-Sharpe gross strategy is regularly the worse net one, and that comparison is exactly what an objective-function should encode.
Related: transaction-cost-modelling, capacity, objective-function, volatility-targeting