Because it represents the latest CLOSED candle, which means that if a cross of any indicator occurred in that candle it will stay that way (in vast majority of cases). Therefore if you created indicator keeping that in mind it would not repaint (assuming no other tricks) and if you created EA a signal would be more reliable. However if you use current candle two indicators might cross each other and then "uncross" each other within that same candle especially in the case of high volatility and/or small period of the indicator (if applicable; here applicable). As a result what you will see at the end of the backtest is EA placing an order in the situation where you can see clearly that a cross did not occur. But if you look at the same order while examining backtest tick by tick or even minute candles (for higher time frames) you will see that the cross occurred for short period of time, EA placed an order and then indicators part ways.
This behavior can result in generating a false signal, unless you are ok with that and your strategy only requires cross no matter how it happened.