Think of using an additional indicator or two on a chart as like going to a different doctor to obtain a second opinion. You are not too sure or clear about what you see on the chart and you simply want to double check before committing (or not committing) to the trade.
Take a look at the chart below for a visual…
A candle has closed below trendline support on this 4 hour chart and while this is a very bearish sign, you would like to back it up with some additional confirmation. As we look at the MACD and the RSI indicators on the bottom of the chart we can see that both of them are showing very bearish signals as well and that would confirm our initial thought about this pair.