Why upgrade By-The-GB plan from $12 to $15 if both have 5G?

Our family is on the "old" $12 by-the-GB plan. Our phones all show a "5G" coverage. But the Xfinity site implies that we need to upgrade to the $15 version in order to get 5G - so that's confusing. I'm unable to see any tech details at all about the $12 "old" plan so I can compare all specs - maybe there are other reasons to upgrade.

Does anyone know what the real advantage is to upgrading? I found a forum post from 3 years ago with a little info, but up-to-date info would be nice and I thought someone else might have more information.