the advantage is to for a general reason of adding more capacity to the system that's the short answer.
the long answer is Reserve Capacity and Cranking Amps, when you run batteries in series the voltage adds but the amps stay the same, when you add batteries is parallel the voltage stays the same but the amps add, so in this case for every two you add to pack you get more amps while maintain the same voltage.
the math for simplicity lets say 12v battery with 1000 Cranking Amps
so two batteries in series 12+12=24volts 1000amps total,
two batteries in parallel 12 volts 1000amps+1000amps= 2000 amps (this is how you would jump a dead battery adding the needed amps but not change the volts.)
so two in series 24v 1000 amps . two of these sets in parallel is 24v 2000amps (this is your 4 battery setup in a 24v truck)
add two to this set up and you get 24v 3000amps( this is your 6 battery setup in a 24v truck)
this also works for Reserve capacity theoretically you can run engine off loads longer with more batteries in the system as you have more amps to play with.
This is theoretically true, but I question how much practical truth it holds. The battery box is still the same size, so changing the configuration of the same volume of batteries inside shouldn't really create a meaningful change. It's like replacing 2 D flashlight batteries with 8 AAA's. There are nuances to this, such as the rate that battery can chemically convert the stored energy back into power on the cables, the rate at which more batteries will discharge themselves faster when sitting due to the internal resistances, the maximum current the cables/hardware can safely handle, etc., however, I don't think there is a lot to gain (if anything).
Even though batteries are rated in Ah (Amps * Hours) of reserve capacity, it's not strictly true. The faster the battery is discharged, the less Ah capacity it will actually produce. In other words, more smaller batteries will actually get stressed harder and produce less duration of output. If you have a 100 Ah battery, and discharge it at 0.1A, you'll get way more than 1000 hours of output. But if you discharge that battery at 200A, you will not get a full 30 minutes.
Almost any combination of batteries you stick in there will work, in a basic sense. But somebody that engineered these trucks sat around for a long time calculating needs and choosing batteries. So aside from wanting batteries that are easier to buy locally, I'm skeptical of claims that these smaller batteries are "better" functionally (even with more), until I see some hard data.
