They scan your MB to enter the park. That's it. There is no FPs so no scanning for those (unless you have a BG for RotR they will scan for that). You can't charge to your MB if you're off site. Do they scan MBs if you have an ADR somewhere? I have only been eating at counter service places with mobile order. Just have them scan your ticket. I personally wouldn't pay extra to get a MB.
ETA--I forgot the photographers will scan your MB so are you doing a Disney picture package? Also the ride pictures are automatically uploaded to your MDE account thru your MB or when you scan at the end of the ride.
For offsite guests, the main advantage of having a Magic Band is mostly the convenience of not having to take your ticket card out of your pocket. Since FP+ is not currently in use, the only time you'd really need to scan it would be at the entrance, so there really isn't a need for them right now unless you care about ride photos as there are a couple rides (HM & 7DMT) that do require a Magic Band for ride photos.
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