It helps in instances where the console can't supply enough power to your Xim-connected devices. You can have power issues due to kb LED lighting, rechargeable wireless mice, an undercharged controller, or due to long USB cable runs. These symptoms are most common when using Razer mice and keyboards...so maybe don't, they're not good quality.
If your controller suddenly disconnects, or you get stuck in a strafe, or circle around looking at the sky, or you need to reconnect your Apex every 5 minutes, your gear is too power-hungry for the console and needs some supplementary power.
In short, unless your Apex stops working, you don't need a power supply.
