止损是投资中非常重要的一环，它可以帮助投资者在股票价格下跌时控制损失，保护投资本金。以下是一些为什么需要止损的原因：

1. 控制风险：投资股票存在风险，市场波动和公司业绩不佳都可能导致股票价格下跌。通过设置止损点，投资者可以限制自己的最大损失，以保护自己的投资本金。英文翻译为Controlling risk: Investing in stocks carries risk, and market fluctuations and poor company performance can cause stock prices to fall. By setting a stop-loss point, investors can limit their maximum loss and protect their investment principal.
2. 避免过度损失：如果投资者没有设置止损点，当股票价格下跌时，他们可能会遭受更大的损失。这不仅会影响他们的投资回报，还可能对他们的财务状况造成不利影响。英文翻译为Avoiding excessive losses: If investors do not set a stop-loss point, they may suffer greater losses when stock prices fall. This not only affects their investment returns but can also have adverse effects on their financial situation.
3. 保持投资纪律：止损可以帮助投资者保持投资纪律，及时卖出亏损的股票，避免情绪干扰。通过遵循预设的止损点，投资者可以更加客观地做出投资决策，减少投资失误。英文翻译为Maintaining investment discipline: Stop-loss can help investors maintain investment discipline, sell losing stocks promptly, and avoid emotional interference. By following pre-set stop-loss points, investors can make more objective investment decisions and reduce investment errors.
4. 保护投资组合：止损可以保护投资组合免受个别股票大幅下跌的影响。通过将投资分散到多个不同行业和公司，投资者可以降低整个投资组合的风险。即使某个股票出现亏损，其他股票的盈利可以抵消损失。英文翻译为Protecting investment portfolios: Stop-loss can protect investment portfolios from the impact of individual stocks falling significantly. By diversifying investments into multiple different industries and companies, investors can reduce the risk of the entire investment portfolio. Even if a particular stock experiences a loss, the profits of other stocks can offset the loss.