A must have if you are running a second battery, especially if you are running lithium cells or any secondary battery battery. It can save you the trouble of to dead batteries or overcharging lithium batteries.
Pros smart auto connect and disconnect of secondary battery from charging system which isolates it m from other battery. With it disconnected from main battery if somthing pull you voltage down it will not effect the secondary battery. Or trash lithium banks by pulling them down and not being able to charge them back up. Oh it also comes with a thin almost flat switch that you can manually connect both batteries together to jump start yourself.
Small Con for some, bit pricey compared to others but still a lot cheaper than replacing a bad battery.