Trying to decide if I need to get one. It seems that everyone at the track has one. Why do they use them? I currently use 2S and 4S lipo's using an Onyx 235 charger.

Short answer ..

The number one reason is to power a charger. Some chargers are DC only and require a power supply. Other chargers benefit from being able to charge at higher amps.

Your Onyx can only charge up to 5 Amps while using AC input, if it was connected to a power supply it could charge up to 8 Amps (assuming the power supply is rated above 8 amps). Charging at a higher amp rate decreases the time it takes to charge your pack.

Got it. That helps. Thanks.

The better chargers are DC only. Better charger usually means higher amperage, and can maintain that amperage longer through the charge. You also get better features out of higher end chargers like cycle charging, discharging (and at higher amperage), internal resistance checks, etc.

I would not get a PS less then 20 amps, and would go higher if you can afford it. I have never used the computer power supply conversions, but they are inexpensive options as well for good amperage.

I use a computer power supply conversion and it works great. It was super simple to build and only took about 5 bucks worth of parts to build.