So I'm doing research to possibly buy a new F250. Looking at the window stickers, I see some that come with dual batteries and some that don't. Why would it come with dual batteries? What is the purpose?

 

A vehicle with a diesel engine always comes with 2 batteries.
Gas engine vehicles having special features requiring more power than normal can come with addl batteries.
Witch are you looking at?

 

7.3 Gas.

 

It depends on what you are going to do with your F250.

Let say a 2024 F-250 uses Motorcraft BAGM-65-A, when it comes with dual batteries, simply BAGM-65-A x 2.
Ford with factory dual batteries always use parallel setup and the major specs are: 750 CCA, 68 AH, 130 RC
So you will get:
CCA: 750 x 2
AH: 68 x 2
RC: 130 x 2

In reality let say you are located in cold area you will want more than 750 CCA on single battery but 1500 CCA is overkill.
Now let say you will run an inverter and power quite a few power hungry accessories when engine is off, you want higher RC, 130 x 2 is insufficient.
If this truck is not going to be your daily, and you only drive it once a week, you will want to add a solar panel, dirt cheap these days, check the custom setup link to another thread below.

How good is BAGM-65-A? 3rd worst rank 8 on the list below:
Ideally if Ford gives you dual batteries on XS Power shown below that'd be great but there is no way Ford would do that.
Why bother with two low-end batteries when you can upgrade to a high-end battery now or later on when it dies or do a custom dual battery setup (isolate them) and get a true deep cycle battery on the 2nd one.
When you run power hungry accessories ideally two XS power is NOT better than a single XS Power + Renogy Deep Cycle 200AH but the Renogy, due to its different in size, has to be mounted elsewhere. One of the advantages of running custom setup and isolate the batteries is you don't have to replace both at a time like you would do with factory dual batteries setup. If you want custom setup have a look at this thread here.

1) XS Power D6500 ($442.62 at ebay): 968 CCA, 86 AH, 174 RC
2) Odyssey ODX-AGM65 ($403.99 at oreilly): 950 CCA, 74 AH, 145 RC
3) Fullriver FT930-65 ($567.36 at ebay): 930 CCA, 74 AH, 142 RC
4) ACDelco 65AGMHRC ($148.79 at rockauto): 775 CCA, 70 AH, 150 RC
5) Bosch S6551B ($225.79 at rockauto): 750 CCA, 75 AH, 150 RC
5) Super Start 65PLT ($244.99 at oreilly): 750 CCA, 75 AH 150 RC
7) Odyssey ODP-AGM65 ($348.99 at oreilly): 762 CCA, 64 AH, 129 RC
8) Motorcraft BAGM-65-A ($141.79 at rockauto): 750 CCA, 68 AH, 130 RC
9) Duralast 65-AGM ($244.99 at autozone): 750 CCA, 68 AH, 120 RC
9) Interstate MTX-65 ($274.95 at ebay): 750 CCA, 68 AH, 120 RC

Look at the list above, if you are on a budget, ACDelco appears to be the winner. Interstate brand normally has a good reputation but unfortunately not on group 65.