A smart car power adapter charger is useful when you have a lead acid battery permanently attached to a charger. The most important thing that a smart car power adapter charger does for you is that once the battery is fully charged, it switches to 'float' charge mode that maintains the lead acid battery in a fully charged state without over charging it. If you have a vehicle you do not use often, then you might attach a standard charger every couple of months to keep it charged, but with a smart charger you can leave it attached permanently and maintain your battery in a perfect state of car power adapter charge.
The better car power adapter chargers will have a power mode that supplies 13.8 volts so you can be running your 12v system and it will supply the charge through the battery. The average "dumb" charger does not charge very well if your running stuff from the battery.