why does my 2002 E250 gas motor have two batterys

It's either an aux battery for the engine, to help start (sometimes in VERY cold climates folks have aux batteries so the van will start at -40) or an accessory battery that is not used in starting but runs other things like a motor home and charges when engine is running. Look and see where the wires lead

25 years ago I had a motor home. It had 4 batteries. 2- six-volts hooked up in series, 12 volts to have plenty of CCA's.. Then a 12 volt battery for dry-camping, deep cycle--can run dead and not damage it, then charge with engine running. Then another 12 volt, independent of others to make sure never got stranded. But Onan generator, one pull start every time without fail with a 12 volt charger would also charge them all. When dry-camping, in the days when cell-phones were the size of a loaf of bread and signals rare, remote places I went you DEFINITELY don't want to be stranded, but use your lights, music water pump and not have to worry about dead batteries,